I also have pectus excavatum, though I haven’t had surgery for it (but it was apparently one of the deeper pectuses (pecti?) that some doctors had seen), and have also wondered about how it might be impacting my cycling performance.
I seem to be slower than a lot of people who train less than I do, but that could be due to other differences - I basically hated any sort of sports until I started cycling, so I don’t have as much of an aerobic base established as people who’ve played sports all their life, for instance. I’m 25 now, and I’ve been cycling for about 3 years, and doing some sort of structured training for the last two. In 2018 I had plateaued at around 215-220w, doing a lot of unstructured riding (and probably resting too little). I became more diligent about recovery and structure in 2019, doing SSB and part of SPB (mid volume, with some other rides) and got up to around 240. Since then I’ve fluctuated between 220 and 250.
Long sweetspot and threshold workouts are my preference as well. It could just be the positive feedback loop of being good at what you train, but sustained subthreshold work has always been a relative strength of mine.
I suspect that limited chest space is only really impactful at high intensities, and probably more so for the heart than the lungs (I think I recall from the Kolie Moore podcasts on VO2max that the long-term limiter is typically heart stroke volume, not air intake rates or uptake rates by the muscle - but that might not be true if lung volume is significantly reduced. On the other hand, heart volume is also restricted, so maybe it’s still the limiter). The obvious compensation for a low stroke volume is a high heart rate - curious if your heart rate is on the higher end of what’s typical at various intensities - mine certainly is: zone 2 is around 155-160, and threshold is around 188-190. The highest I’ve seen is 204, but I’ve had poor luck with heart rate sensors giving reliable readings above threshold.