Am not an expert.Just sharing my views.
1.Yes of course it's possible to install after market muffler on GS.
2.Yes performance & a lil fuel consumption might increase.
3.KHI members can guide you better.
4.Whatever you do with ur GS.Keep stock muffler with you even if u install a super sport muffler on it.
5.It totally depends upon it's fitting & the muffler.
6.Totally depends upon muffler u like to have.
I have seen many RP Storms with after market shiny mufflers,giving good sound.
I.M.O a good quality muffler,having good tone can be sourced for 4-5K.
Don't know about KHI market.
You can also find a branded muffler since you guys have option of "ranchor line".