I've used a few DIY Rex reactors and never ran beyond 10 psi, with no problems.
If you drop the pressure down, you'll definitely have to tweak the needle valve, and bubble count should differ as well: higher pressure means more CO2 squeezed into smaller bubbles. The air on either side of the bubble counter presses against the water in the bc. Water doesn't change size based on pressure applied to it, but that pressure is transferred through the water to the bubbles inside it, compressing the gas pocket, meaning more actual CO2 per given bubble size.
That's the theory, anyway. It took me a while, reading through some long arguments here on the forum (there have been a few doozies over the years) to come to believe it. Don't take my word for it - do some reading and come to your own conclusion. But that's my take on it.
In any case, regardless of reasoning, watch your drop checker, and your fish, and gradually dial in your needle valve all over again.
...and I've heard a few horror stories about the ISTA reactors. Just build your own and be done with it.