Rastoff
US Veteran
I've heard this a lot. I've even watched a few guys struggle with it. In every case it was technique that was the problem rather than equipment. I've never had trouble inserting a full mag with the slide closed.You couldn't insert a loaded mag w/ the slide forward. Now I can.
Here's how I do it...(yeah, I know this isn't an M&P, but the technique works with any gun)
First, index the mag:

There are several advantages to doing this, but mostly it helps you line the mag up with the mag well.
Next, put the flat of the magazine against the flat of the mag well:

Use a slight angle and this will be easier.
Finally, in one smooth, firm motion, insert the mag using the heel of your hand:

Where most have trouble is by using a two step motion. They insert the mag a little and then whack it with their hand to insert it. I can't explain why this doesn't work, but it doesn't. It also takes more time, but that's another subject.
Altering your mags could be a felony. So, I don't recommend it. However, in all the time we've had the 10 round limitation here in CA, I've never heard of a person even being cited for it. That doesn't make it right, it just tells us that it's not something the cops are looking for initially. I'm sure it would be tacked on as an additional charge if you committed another crime though.