I’ve been told that on a flat course like Florida I could shave 20 mins off my IM bike time(6-6:30) by swapping my rear kysrium for a disc.
Is this about right?
I think 20min is a bit much, but it is a faster wheel than a kysrium. Use the disk but bring the kysrium just in case of a hurricane.
20 minutes could be achieved by good bike training and a Renn disc wheel. Since you have a perfectly capable pair of wheels, the addition of a Renn disc would not set you back much (under $350), and give you a wheelset that will take you anywhere to any race.
I notice a big difference when I put on my disc…about 3-4mph average for sprint tri’s. The course is flat, very flat! There are only a few rolling “false flat” hills so my disc will be on…unless a tropical storm drops in!
Another advantage of the disc wheel is that advertising messages for your sponsors is easier to put on the disc than on spokes 
I like my Renn disc, and use it even in the mountains.
I use a disk and I like it fine. I still have yet to prove to myself that it’s faster. Yup it feels quick, especially in strong headwinds, but the results show no drastic improvement. No, I have not done any controlled tests. But I have ridden a few courses (race conditions) both with a disc and without. My times don’t reflect that I’m faster on the disc. I still use it. But if you’re one of those people who needs to see some actual benefit in order to justify your purchase, I haven’t seen the justification in my case.
So to get to the point, 20 min IMHO is BS.
My disc experience vs. standard wheel like a (K)syirum is about .5mph on flat/rolling courses. Not 20mins in an IM, but not something to sneeze at either (unless you’re allergic).
The roads in IMFLA being okay, put on a good tire, jack it up with as much pressure as possible, let it roll.
Pooks is correct, 20 minutes is not right. MAYBE a minute every 40 km? That would be about 5 minutes over an ironman course…?
IF the disc added .5 mph; it would be about 6 hours to do the course at just over 20 mph, meaning you would reach the finish line about 3 miles ahead of a non-disc rider…at 20 mph, that 3 miles would take 9 minutes ride, so somewhere between 5 and 9 minutes might be about right for a disc benefit over a regular spoked wheel. I’m just talking off the top of my head, I have no personal testing experience except at shorter, faster distances…and, for me, at the shorter, faster (24mph and up) distances, the disc wheel gives noticeable improvements in elapsed times. I don’t know how much difference there would be at 20 mph…a fairly good ironman distance speed in my book!
If you have a deeper dished wheel to begin with, or a lessor numbered spoked wheel, the differences would be less.
I have definitely noticed an increase in speed with my disk. About.5-1 mph. I feel like I’m working harder going up hills but my splits on hilly courses were faster too with the disk. And most important, you make that cool sound at speed.