Tazz,
Yes a bigger pinion would be the answer to gearing the car for higher speed. As Ari posted, motor temperature is the usual guide to how far you can go with gearing.

Again as Ari suggested, talk to the guys at your local track and they will generally use either of two measurements when talking gearing, those being Final Drive Ratio (FDR) or Roll out. Different chassis have different internal drive ratios so you are not able to compare the same pinion and spur numbers between your X Ray and a similarly specified Tamiya 415 as an example. Hence the FDR or Rollout comparison, as these are able to be compared across chassis makes and types.

I run a T2 ‘007 and the internal ratio is 1.7, and I think yours might be the same, whereas the Tamiya’s are more like 2.25.

A handy web based tool is Gearchart (Gearchart.com). Go to this website, click the gearchart creator and input you car details. Tyre size for EP on road us usually 63mm. When you click the rollout or gear ratio tab a table will be presented for that measurement. This is really handy as you can see a range either side of the spur/pinion combination you entered.

Then ring the guys at Northern Beaches Hobbies and they will post you all the Xray bits you need – in a heartbeat. I ordered mine on Monday and they were waiting for me at home on Tuesday night. Sorry about the shameless plug but they do sponsor my local track in Sydney.

I hope this helps,
Brent