I would assume the thermal dets on the TIE bomber would be larger versions of the hand held grenade, but not an orbital weapon. The bomber is designed to get up close and personal. If you wanted to attack from orbit, use a Star Destroyer, or better still a Death Star, but only if you've got one handy.
I do like the idea of asteroid removal/destruction, but if you shoot something like that, it's going to create debris, just like the Atari game from so long ago. Shoot a big one, get two or three small ones. Ep V saw the Imps blasting asteroids in an attempt to get to the Falcon. Ep II was another example of how to create debris. It seemed George Lucas loved the asteroid fields and the hazards they created for the heroes.
Playing a 50-60point chase through an asteroid field on a larger table (6' x 3') could be a lot of fun. Doesn't have to be an asteroid field either, could be a debris field from a destroyed starship with big hunks of hull floating around. The options are endless. Probably wouldn't limit it to secondary weapons though, some of them are one shot and it's used, and that wouldn't last long enough.
Hastings, New Zealand
Rebels: 3 X-Wings, 1 Y-Wing, 2 A-Wings, 1 B-Wing, 1 YT-1300 Imperials: 1 TIE/Adv, 2 TIE/Int, 3 TIE, 1 TIE Bomber, 1 Firespray-31
and still building...