Ive been looking around here for about two weeks now. Planning on getting an RX8 in the next week or two. One question I had was does anyone know the cubic feet of the trunk without the tire in it? I couldnt find this info on mazda's site.

Yeah, Mazda likes to say how it fits two golf bags. Try edmunds.com. If they don't have the info, probably nobody will.

Trunk volume is 7.6 cu ft.

Can't tell you how much the reduction is in cubic feet (-3cu ft?). In real terms, if you were able to pack 6 soft duffels without the spare, you will only be able to pack 4 of them with the spare. Other assorted items can be fitted around the spare. The primary difference between the trunk with or without the spare is that you won't be able to fit a single large item into the trunk because the spare intrudes into the opening.

I can fit a 2x12 line 6 guitar amp, two Les Pauls, a floorboard, and two overnight type bags.