How much weight training are you guys doing? At a 28 second event, that's a decent amount of anaerobic system. If you guys can get Lionel's swim start into this range, I think he is set. I really think his 50m speed is a limiter in triathlon pack swimming. The dude has plenty of endurance.
I'm not quite as fast as these guys, but close (28.9 and 29.1 the last two times I swam the 50 fly). I do zero weight lifting. I do swim 3-4 times a week, occasionally more. I do swim lots of hard 25's, although not that many butterfly because the quality drops off fast. When I have a meet coming up where I'm swimming the 50 fly, I'll start, about a month out, to do 25's fly on 2:00 at the end of my workout, trying to hold as close to 14.50 on every repeat as I can. When my times start going over 14.80, I quit. Usually can get to about 8.
I very occasionally do something similar for freestyle, trying to hold under 13.75. More frequently, I do 25's free on :30, with a hard target of 14.50 or better. When I miss, I rest a minute, then go again. Go until I miss 3x total, or 2x in a row. Can get into the low 20's on that set.