Just got my crystal oscillator circuit built and installed onto my block erupter.
Without changing the voltage I can get stable output with a 13.5mhz crystal

that is 378 mhz. averaging 350-360 mh/s
Still need to test more.

1 clock is 1 hash, so 378MHz should give almost 378mhash, minus errors.
I get 335.8 displayed in CGMiner with my stock sticks.
So there is something wrong with yours or you didn't wait long enough...