Triathlon is 3 sports and the gear required is different than for just one sport. Your idea doesn’t work and is not practical

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I think the Castelli savings claim is vs. non-aero road jersey similar to most are wheels claim comparison to slow box rim and spoke combos such a Mavic, etc. Your savings over a good tri suit or tri specific aero suit likely won't be as much.

You'll have to test clothing with your setup in the tunnel or on a velodrome to know for sure. My LG M-2 setup was as fast, maybe faster, than my Castelli BP2. The M-2 has generally tested well and with the top configuration you can roll it down for the swim and pull it up it T1 ahead of the bike. If you're male it does mean running with it up.

I find it difficult to believe that someone would save 21 watts going from a good tight fitting tri suit to a Castelli speed suit. Maybe the difference would be near that much if you had a baggy jersey on.

Personally I wear a tri suit that swims as well or better than any swimskin I've tried and I feel comfortable running in it as well.

I could understand how some people wouldn't like to run in a tri suit though.