Hi Matt, I am in a similar situation as you. I have resisted the advice to go for an aftermarket ECU despite the superior results one would achieve. My JDM 90T was bought cheaply with the intent of teaching my 15yr old(now 21) about cars. In about 4 hours it goes back on the dyno to check fuel ratios, since replacing the 30yr old wastegate actuator with an adjustable preload Kinugawa, hopefully the AFRs have improved. Previously would only boost to 6psi, now I've had to adjust the fuel cut higher, so I know I'm getting 12psi. It is an interesting exercise, but my final goal is about 240 -250 rwhp. If I was thinking >300, I wouldn't waste time with a stock ECU. The flexibility and ease of diagnosis and adjustment of both timing and fuel trims is impressive not to mention crisp acceleration throughout. I suspect you will need that eventually so why not now.
Just my opinion.....