FI is definitely better, but you're still going to lose major amounts of power at altitude on any normally aspirated engine. It's simple physics, antd there just isn't any air to be had unless you pressurize the intake.

We ran the national circuit doing TSD rallies many years ago, and one event went over Mosquito Pass (+/- 11,000') in Colorado. My trusty fuel injected 22RE powered 4Runner would top out at about 4,000 RPM in 2nd climbing the 7% grades near the top.