the amount an engine compresses its volume is measured by how much volume it sweeps through the compression ratio... on the wankel, there is a fixed maximum because of the inherently fixed cross-sectional geometry (when you look at it from the side, the peritroichoid (sp :p) and fat triangle must be at that fixed proportion)... the compression comes from the rotation aspect of the rotor's orbital movement (where it both rotates around and orbits around the outer gear (which is inside the rotor))... hope you're not confused yet
here is a link to an animated gif which will illustrate this best (i'm not taggin it out of respect for the copyright):
reducing the compression ratio is easy, yes, by grinding out a deeper dish in the rotor. this is quite bad for overall thermal efficiency (smoking fuel consumption and torque when off boost), and is not as good as making the same amount of torque at rpm (power) with less boost and more compression.
no, there is no way to increase displacement unless you get wider rotors, or an entirely upscaled motor (bigger rotors, housings, etc...).
porting is a seldom-understood black art. it's probably not something the average joe would want to endeavor upon unless building a motor for the track (where the more power you can make within the rules and limits of your engine, the better).
by the way: larger orifaces and flow paths DO NOT equate to "freer flow" or "less backpressure". that is a fallacy.
well, yes, seals can have a large impact on performance... the degree to which they seal, and their tolerance for specific applications is important to consider when building a motor. if you mean to ask "are the 2mm seals in the RENESIS thick enough to take some serious boost", i'd say yes, as they're at least as good as the Genuine Mazda seals in the 13BREW (i dunno if they're the same or not... certianly not worse), and those have seen some extremely sick levels of horsepower, and seal better at higher rpms than the heavier 3mm seals.