Your ESC won't be drawing more amps from 25C batts than from 40C batts, on the contrary, since the 25C battery voltage will sag more under load. You'll get more grunt with higher C batts, but may not be really noticable since the wheels will spin easily even with lower C batts.
25C batts will be harder on the ESC capacitors though (higher ripple).
I'm using 5S 5000mAh 45C Zippy pack from HK and it's a perfect fit.