Put a small enough pulley on to spin it to 19,000rpm and you can hit 18psi. But it takes a lot of meth to combat the heat.

I'd start with a .5mm jet. You will see some IAT cooling but some will be combustion cooling due to the water. If you want max IAT cooling the more meth you use the better.

I think the gauge should work fine, if you have a big issue once running it maybe possible to re-flash your board for better resolution for a single small jet.