Well I've finally concluded I'm not gonna get around to finishing it, so here's an old Wip for those interested plus some known improvements.
http://dehacked.2y.net/microstorage.php/info/674312144/Sonic%20Spinball.gmv
Improvements so far are using only 3 bounces on level 1, and a few frames saved on level 2. Not in the wip yet, but there's a few nice improvements in level 3:
http://img62.imageshack.us/img62/867/sonicspinball000.png
http://img594.imageshack.us/img594/9418/sonicspinball001.png
http://img291.imageshack.us/img291/2759/sonicspinball002a.png
First the much better opening strat, you can bounce off the left flipper, into the right bumper and that'll get you down that left hand tube a lot faster, and with some initial speed too. For a better refinement, you can clip off that red shocker thing for even more speed. In fact, if you do this properly, you'll have enough speed at the bottom to be able to bounce straight up to the above flipper, without using those steam piston things, which is always nice. Haven't found anything else in the main route, but the boss can be improved by launching sonic over the machine and getting him stuck bouncing parallel to the tube.
http://img185.imageshack.us/img185/8903/sonicspinball003a.png
Finally on level 4, a better method for getting the emeralds on the sides. These techniques have a few quirks needed to be worked out still; as I said I never got round to finishing them, but there's certainly improvements to the current run.