As far as quality..it depends on what type you're talking about. SQ (sound quality) will be better with 2 8s. They will be more punchy, tight and faster response as the voice coils work faster. A 10 will provide with deeper response, but not as tight.
I have 2 10" kicker CVrs in the back of my TJ, and they are more than enough. With the top off and going 75 down the highway, the bass is still very loud, and with the hardtop on, ive had to reinforce my windows lol. Not too sure about the instatrunk and how theyll sound through that though...maybe someone with more expereince there can tell ya