Iam not sure if it would make a difference or stick with 50mm tubulars rims.I not the fastest rider out there but not bad,on a hilly half IM I can average 19.5 mph full IM 18.25mph (IMLP).Does it make any sense to use a disk with my current speeds. I dont think I will be getting too much faster at this point in my life (49)I ask because I may get a disk this winter.My racing future will be 1 IM, 1 to 2 half IM,& olympic per season.Thanks for your input.
You probably won’t see too much of a difference but sometimes just having some nice high end technology on your rig will give you a mental advantage with yourself and you might go a little faster, kind of like a placebo.
Rent one for a race and see if you like it. www.racedaywheels.com has some very nice high end wheels to rent
I vote Disc. It does make a difference. Whether that difference is significant or not depends on your racing goals. Once caveat, as long as it does not stress your bank account. Plus, you can use it in shorter races or TT’s.
Scott
A disc is the fastest wheel. Another option would be to get a $100. custom cut disc cover from wheelbuilder.com to use with your 50mm rear.
If nothing else, the disc and a fast front wheel will make you go through the air faster. I mean, Pinewood Derby cars, going a whole two miles per hour, benefit from aerodynamics.
I can’t remember the formula for this, but after a certain mph, you have to work ever so much harder to go a mile faster. When I am going 20 mph on a full-on aero bike, my perceived effort is like I am going 18 mph. Doesn’t sound like much, but over a long period of time, you save a lot, especially for the run.
Why not make your time on the bike easier, if not faster?
Yes, I would go for the new Zipp Sub-9 disc. You can get an additional 11 watts of power in certain wind conditions. Life is too short not to get it. Its not going to break the bank so go for it.
Yes, you will be faster with a disc and save more time at your speeds than a rider who avgs 23 mph for an IM. But, do those 5 mins (WAG) matter? The disc roars when cornering, is a pain in the arse to inflate, and doesn’t ride as nice as a spoked wheel. Does make a really cool whoop-whoop sound when pedaling though that a cover does not 
Is a cover worth the money - no doubt yes. New Zipp disc? Doubtful for you. Renn 575? Sure - go for it!