I have notice the c latency can be as low as 4us I first thought that was a glitch but it happens fairly often now that I'm looking , so the odds are that increased complexity in the isr will spread the range further.

there is a thing called chipkit-pi a pic 32(28 pin dip) clone of the arduino , its designed to plug directly into a raspberry pi and has a free ide based on the arduino ide . it has some limited arduino library compatability .
that's my next toy , perhaps thats the step up past 128k ,midrange c gets you nowhere extra in that regard