Firstly, Welcome to Ribnet!
There are "hard floor SIBS" out there, which although fal lin to the "rigid hull / inflatable category are usually pretty flat bottomed. No idea how large the backof your SUV is, but I do have to ask, how easy would it be to throw the inflated boat (minus engine!) onto the roof? that might allow you some extra space in the form of a bigger boat. If you go fully inflated, most will roll up and fit the back of something big.
One other question for the moment - when you say "not fast enough" just how "not fast" are we talking here? Also how big was the engine on the Achilles? Simply because if you are talking about "trading up" to a 2.6m, I'm guessing you had something like a 2 or 4 HP?