Here's a map (copied from GameFAQs without permission):
.-----------.---.
| | |
| v v
1-->2-->3-->4-->5-->6-->7-->8-->9-->10-->11-->12-->Boss
| ^ ^ ^ | ^
| | | | | |
.-------.---.---. .------>13--------.
The possible paths are:
01 -> 02 -> 03 w> 07 -> 08 w> 13 -> 12 -> end
01 -> 02 -> 03 w> 06 w> 10 -> 11 -> 12 -> end
The former one looks to be faster because level 11 has so many detours. The others (except 13) can be run almost straight through without deviating much.
Of course, the only way to know for sure is to make two runs...
Edit: I finished up the first level, 10 seconds faster total.
http://fast.filespace.org/blublu/SolarJetman_U_level1_finished.fcm
Edit: Level 2 done, now the total improvement is 22.95 seconds. :D
fast.filespace.org/blublu/SolarJetman_U_level2_finished.fcm
(obsolete)
Edit: Re-did part of level 2, improving the time by 1.38 seconds.
http://fast.filespace.org/blublu/SolarJetman_U_level2_finished_again.fcm