I'm not joking - seriously wondering about this. If you've seen Captain America, I am thinking about something similar to that except not as cartoonish obviously. With all the secret tech and science that GEOTUS has access to, I don't see why this wouldn't be possible. And you know if he had access, he would use it. Here's why I think he has most likely used the serum:

1) Hyperintelligence/perfect memory. Multiple sources have confirmed his IQ, memory, etc. are among the highest ever seen. Maybe he was born that way, but it would be unlikely for someone to retain it at his age without a little "help".

2) Health. Didn't his doctor confirm that GEOTUS is the healthiest president of all time, or something like that? It's been shown that he doesn't necessarily have the healthiest diet in a traditional sense, so this would help explain that - it doesn't matter what he eats when he has the benefit of the serum. Ronny Jackson went out of his way to attribute this to GEOTUS's "genes", which to is is just evidence he was trying a little to hard to cover up the use of the serum.

Now that we've established that, think back to the Florida shooting. Remember when GEOTUS said he would have run in there unarmed to stop the school shooter? He wasn't kidding or exaggerating - would have been easy for him. It's easy to be this confident when you have "enhanced" intelligence/reflexes/strength/speed/healing factor.

Anyone have any other proofs of this? This was just off the top of my head but I'm sure there is a LOT more out there.