Maybe my n=1 will help. I did IM Boulder after 6 months of training at sea level. I'm new to tri (those were my first 6 months in any of the sports) so I don't have reference points indicating how much the altitude slowed me down, but I didn't consciously notice it at all. The race itself felt no different than training in my hot coastal town. Overall my swim time was just barely slower than expected, my bike was right on target (maybe even a bit fast), and my run suffered a little but I think that was due more to the ridiculous heat. Putting it all together I nailed my sub-11-hour goal by a decent margin.

The thought of altitude stressed me out for the first few months, but then I just got in the mindset that 5400 feet isn't a huge deal. During the race I paid a lot of attention to RPE and heart rate, and stayed within myself. The altitude was fine, and I really enjoyed being in Boulder. I'd recommend it.