Don't mess with the stock spring. Unless you plan on shooting some exotic ammo. It's a 9mm. My 10 y/o daughter can handle some of the hottest 9mm loads with a stock RSA w/o a problem.
The 2 things I make sure all my Glocks have are night sights and factory extended slide locks. Why doesn't Glock just start selling with the ext slide locks I will never know.
The only other mod I did with my Glocks was the Glockmeister trigger upgrade. I liked what it did to my Glock I did to the all. It's smoother and lighter, around 4 1/2 lbs.
And I don't think it's out of the ball park to compare to M&P. I think they are comparable when you look at functionality, reliability, and accuracy. Where the Glock has a huge advantage when it comes to availability and cost of mags, holsters, interchangeability, parts, armorers (if you need one) ease of disassembly, choice of calibers, convertible.
The G26 is the most popular concealed carry piece in the world.
Well... I don't know if it really is but should be. It definitely is the most popular CCW in this country.