I see what you're getting at, but look at the crappy drawing I posted up again. From N to S on the stock setup, almost the entire core is covered with coil. On the HPI setup, only a portion of that core is covered with coils, as the core basically goes half-way around the stator.
If you can grind a tomos A35 core to fit snugly against the HPI core, that would be good, but since the core has a bigger cross section, you simply can't wrap as much copper around it.
I'd almost be tempted to go the other way (smaller core), to get more copper wrapped around the core.
The relationship between RPM and output power is pretty linear for the HPI setup (through at least 12000RPM), so the core is nowhere near saturated.