No I don't think you HAVE to shoot heavier to get more speed. With the slower bows it helps. Spine? My 340's are still 'bullet hole' with the extra 6# and noticeably faster even though the increase should have only been 10-12 fps. I wont shoot a real light arrow, too much stress on the bow.
The 340's is what I have (2 doz of them) is all I will have for a while. Shooting a lighter arrow works yes, but it means $$$ which I'm saving for another bow (hopefully). I don't see any number of tweaking with the M-Pro picking up enough speed.
If you cannot handle the weight? Yep shoot a faster bow IF you can afford it, if you can handle the difference in where the cam peaks (and holds) in the draw and if you have the opportunity to make the choice. But your right, a bow with cats will shoot much faster than with soft cams all things being equal. Sometimes though, there is more to how you choose a bow than the mechanics of it all.
OK, now I have a better understanding of why you made the choices you did. I do respect those choices, too. I do have to admit that you guys in Aussie land and Europe really have some high prices for archery gear.
If You're Not Living on the Edge You're Taking Up Too Much Space
Martin Gold Plus Staff Shooter
PSAA Life member, UBP Life member
PADI AOW Diver