Measuring the cylinder pressure is what needs to be done.
I agree with this. Until you know what cylinder pressure you have, you won't know which way to go for improvements. I'd also ping the builder to see if he can provide cam specs. If it was built with stockish compression (the overbore lowers it, btw, if you use stock height pistons) and he put in a bigger performance cam, then you have a recipe for sluggish response due to low cylinder pressure.
If the cranking pressure is low, the best stealth approach may be to swap cams to one that build more cylinder pressure. more pressure = more power.
However, by all means, dial in the carb and timing. That is never a wasted effort.
