I've used the T.H.E. extended, uncaptured guid rod on a G-34 and G-17L.
The 34 needed a 15 pound spring for practice ammo, and the standard 17 pounder for hollowpoints.
For some reason my 17L will run fine with a 20 pound recoil spring, go figure. (I use a 17 pounder normally)
Maybe the spring weights aren't exactly the same from brand to brand, or there is some other mechanical factor, it doesn't really matter. Try a 15 pound spring, I always used ISMI flat springs from Custom Glock Racing. They have some tech articles there on spring weights that is very informative.
How far was your brass being ejected with the stock spring? I suspect your load is not the hottest, and they were just getting out of the gun with the stock spring.
My longslides run real smooth with the tungsten, and it seems to reduce recoil as well. Don't tell anyone, but occasionally I use a recoil buffer too. (and a brass slug in the grip, shhhhhh....)