I saw something awhile back where a company called Pearce has baseplates for standard size M&P 40 magazines that increase the mag capacity from 15 to 20 rounds.

Anybody know about this?

I want to buy some used M&P mags and convert them to 20 rounders.
And if not Pearce.....any other companies?

I've seen their '+1/+2' extensions, but not a '+5' extension.
Have you checked their website? If they have one, it would be listed there.

You might check out Carver Custom or Taylor Freelance. I bought T-F extensions and they work with both 9mm and 40 S&W. I'm able to get 28 9mm rounds and 23 40 S&W rounds in the 170 mm extension. T-F also makes 140 mm extensions and a couple of others with round capacities similar to what you mentioned in your post.

Check out Taran Tactical. That have a few different ones. I have one of their plus 4’s on a 17 round 9mm mag making it a 21. I can fit 24 in it but it’s extremely tight, I only shoot it with 21 in it.