Can't comment on your spring but I have run an ISMI spring before.
I purchased an ISMI spring from G&R Tactical over a year ago.
I counted 32 turns if you don't count the ends. It's a stainless alloy and chrome silicon type spring. Cost me $20 and whoa nelly does it makes things stiff. Pulling back on the chargind handle requires a noticeable amount of additional strength. I figured this would be a good thing as it would slow down the carrier. Well it certainly did that! Running this spring in conjunction with an MGI RRB and an H&K mag caused short stroking after EVERY ROUND. Switching to USGI mags caused it very infrequently but enough to make me re-think all this heavy buffer and spring business.
I got rid of the H&K mag because that's plain bullshit that a $10 USGI mag out paced this $50 dollar wonder shit mag. I of course STOPPED running the ISMI spring in favor of the factory carbine spring. I held onto the spring because I think it'll be better suited in taming the carrier on a 9mm build.
It should be noted that, running the ISMI spring, and a regular 3.x OZ carbine buffer ALSO short stroked with that H&K mag. Needless to say both the mag and that spring aren't running anymore in my Bushy.
-Moco