In the simplest terms, yes, backpressure is bad, and less of it theoretically means more power. But OEM exhaust systems (as well as aftermarket ones) are typically designed, tested, and re-designed to maximize ‘scavenging’ and exhaust velocity, which can have just as much if not more affect on power under certain circumstances (ie on turbo’d vehicles - the higher the exhaust velocity, the quicker the turbos spool)
The tech was right in that experimenting can go either way... and while gains at certain RPMs are definitely possible, you could also lose power (especially low end torque.) I’d be more cautious of a tech who promised any actual gains beforehand, given the untested performance aftermarket for these vehicles.
That being said, I whole heartedly support anyone who wants to test new mods in the pursuit of power and style. That’s how a community figures out what works and what doesn’t, one mod at a time