1) My issue with the GG straps isn't really the fact that they're so wide, but that they're wide and extremely close-set at the top end on the pack. I measured the gorilla straps and they were literally like 1.5-1.75" apart (inside edge to edge), where they connected to the upper back panel. This is the real culprit in regard to the neck and shoulder pain that can cause, IMHO.
2) I didn't find that to be in issue with the Starlite if there was actually enough volume to fill the thing, but seriously, this pack is HUGE, I really don't see how the Mariposa Plus can really be as big.
3) Hard to say, it's really an issue of fit, the torso of the GG Gorilla (not exactly the MPP but same frame I think), was way better for me than the Starlite, but my friend that I passed it along to loves it and it fits him perfectly, he says he can barely feel it up to 30lbs.
4) I don't think so, but I'm not certain?
5) The straps are the major gripe with GG packs for me, otherwise I absolutely love them. If you buy a GG and want me to replace the straps for you, let me know and I'll help you with that. The Starlite, while I haven't compared it with a mariposa plus, seems gigantic to me. The pack body is huge, but coupled with the pockets its massive, I could never fill it. Also, the strap /torso adjustment connection, while very novel, caused me alot of pain in the back of my neck, that being said, the Starlite I had was an older one in the blue dyneema, so I'm not sure if it's the same. I also had a problem with weights over 30lbs causing the grosgrain that the straps connected to to get destroyed.
I think the stays in the GG packs are better, it's one piece, and it's got good rigidity, I kept having to rebend the ones in the starlite, and they were just too long for my torso, which is 20".
I loved the idea of the Starlite, and I'm a big fan of SMD's gear, but it never worked for me.