First off I'm a GT86 [FR-S/BRZ] owner and let me say if you're ready to put down the money it is an absolutely fantastically hackable car. If you're only talking about electronics the entire dash is super easy to take apart and it's super clean and organized in back with very nice access to core system lines and power. There are plenty of aftermarket ECUs and ECU extension units and of course it's a new generation Toyota so if you wanted to you could hook up to one of the data lines and play with all sorts of things. ODB-II in the vehicle also seems to have some extensions and I've seen people getting oil temp/pressure from add-on sensors through it. As for performance the ammout of engine, drivetrain, footwork and body mods available for them is insane. Here in Japan they actually have huge monthly catalogues of new parts just for the 86/BRZ. Of course stock is pretty nice as-is if you have a model with the torsen diff (but if you get an R grade a helical would be a superior addition).
As for the EVO vs WRX I'd argue for the WRX Impreza just from the ammount of fantacism that continues around them continuing to drive aftermarket parts production. Case in point: find a bolt on turbo for an Impreza GC or GD - you can get one brand new / an older EVO? good luck. For a used WRX I'd actually recommend looking for something between a later GC and a GG because they were built in an era when constant adjustments and part swaps for different rally types was the thing and also because a lot of the fantics tend to center on those "glory days" models. Of course owning an 86 it's not like I'm baised toward Subaru or anything *cough* *cough*. I'm super curious about the new WRX STI but I've only seen one once and never handled one.
One addition: the Mazda Roadster [Miata]. It's pretty much the "hackable" car of the century for anyone who cares about road handling over numbers (sorry Civic). They're super easy to mod, there are plenty of parts, and there is enough cross-modle compatibility in parts you can pick up scrap spares fairly easily. Just the fact it's a well built, small sized, convertable FR makes me want one for the weekends. Only demerit is apparently in the US they are considered "hairdresser cars"; but a nice beefy rollbar, a spoiler and some badass footwork should get rid of that image.