Yes, I'm already using that trick in Sonic Rush and I believe so are some of the speedrunners. It's actually not limited to boosters, it's something you should take into account almost everywhere in a TAS (and it applies to all 3 games), and it's a logical consequence of how the engine works if you understand it well enough and you know what boosters really do... uh, this is something I know way too much about, and it makes properly TASing Sonic Rush games more complicated than you would expect, so let me write a little essay here (even if you might know some of this already) and maybe this will make more sense:
First of all, if you're seeing speed values like 275, you're not watching the right values. You should see something more like 49152 at normal boost speed. Assuming these aren't dynamically allocated addresses, Sonic's speed is at 0219B30C, his X velocity is at 0219B300, and his Y velocity is at 0219B304. They're 4-byte signed values. (In Sonic Rush these values are elsewhere and are smaller by some factor but otherwise behave in the same way.)
To make sense of what's going on, you'll need to pay attention to both speed and (mainly x) velocity... it might seem like they are basically the same thing, but the game treats them quite differently, and another way of labeling them is "potential speed" and "actual velocity". When you're on the ground, they are tied together directly and act as you would expect. (But keep in mind that what I call the "speed" can be negative to indicate leftward motion, so while you're on the ground it's actually (sqrt(xvelocity^2 + yvelocity^2) * sign(xvelocity)).) When you jump, the velocity and speed values continue updating, but the speed value now updates in a weird way (though the speed value has no observable effect while you're in the air, only the velocity values do, so you can't notice how weird this is normally):
What the speed value does in the air is, it doesn't change at all if you're holding forward (left/right), it decays toward 0 if you're not holding forward (it decays at the same rate as your velocity normally would, even if some external force is holding your actual velocity steady), and if you're holding backward it decays towards 0 even faster (but not as fast as your actual velocity normally changes in this case). Also, the velocity value gets a modification, it's not quite as weird but it's the whole reason the trick works: When you're in the air, if you are not holding backwards, and if the x velocity value exceeds +/- 40960, it gets capped to +/- 40960. So you can avoid the cap by holding backwards, but the speed and velocity values still decay toward 0 when you do that (if you keep holding it, the velocity will decay down to +/- 40960 anyway in only a few frames and then later cross 0 when you fully reverse directions, and then the speed will hit 0 a few frames after that and stay locked at 0). Oh, and if you're boosting in the air then the velocity can still get capped if you're not holding backwards, but it uses 49152 as the cap instead of 40960 in that case. If you hit an obstruction while in the air, both speed and X velocity get cleared to 0, and hitting various level object probably messes with them too, but besides that they stay independent from each other while you're in the air.
One more piece of information about how the speed and velocity values interact is what happens when you land. You can probably guess from the above that those two values get easily out of sync from each other while you're in the air (even the sign of the two values can become different), but they are always the same when you're on the ground, so the game has to do something to combine them back into effectively one value when you hit the ground. What it does is it chooses the larger (in magnitude) of the two values and makes that your new speed and velocity. (I left out some unimportant details of slope calculations, but that's the gist of it.) This explains why sometimes you can start running at full speed the instant you hit the ground even if you were barely moving in the air: when that happens, it means your velocity value is low but your speed value is much higher, so the (hidden while in the air) speed value suddenly takes over when you land and you magically "hit the ground running". I believe that this (and most of the above weirdness) is a deliberate quality of the Sonic Rush engine that exists for the purpose of fixing the problem seen in earlier Sonic games where you grind to a halt if you "hit the ground at a bad angle".
So what do boosters have to do with this? Well, I think you said as much already, but after hitting a booster, your inair velocity update is temporarily modified such that letting go of the d-pad doesn't cause you to slow down at all, and holding backwards does still make you slow down but with a drastically reduced deceleration rate (I'm not sure whether this is a bug or an intentional feature that it applies in the air). Thus, after hitting a booster, you can hold backwards for a long time without losing much of your velocity, whereas without a booster your deceleration is much faster so you can only hold backwards for a few frames before dropping below the speed cap (unless you are going really ridiculously fast, as is possible in Sonic Rush). What this means is that it's often good to jump and hold backwards after hitting a booster, and even after a regular airdash or when jumping on a regular downslope without a booster, you can get ahead a little by holding backwards for a few frames and then holding forwards as soon as the velocity is about to drop below the cap.
But, you have to keep in mind that this is almost always a trade-off, due to the "speed" value behavior I described above: When you hold backwards to exceed the inair velocity cap, the speed value is still decreasing at a normal rate, which means you are gaining (short term) speed in the air but you are quickly losing (long term) potential speed that you could otherwise get the next time you hit the ground. So you have to constantly balance those two things if you want to be as fast as possible overall.
