A few thousand dives so far, and only a handful of leaks... Like, when I hole a glove on a chainsaw blade (happens, but surprisingly rarely - they're pretty tough... And when it does, only the tip of that finger or whatever gets wet). I have never had a leak at the glove/ring interface, and I've never had a leak at the ring/ring interface.
I once had a leak at the ring/suit interface. I was underwater with the chainsaw and in an odd position due to the current and weird cutting angle. My shoulder and elbow were flexed hard and pulling on the drysuit and glove, which forced the seam open a little along the backside of my wrist. I got about a tablespoon of water in through the seam before I jumped (sudden cold spot), and that jump resealed the area and I didn't have a problem again for that dive.
When I got home I pulled the suitside ring off of the suit, cleaned everything up and put it back together. Turns out that I'd had a wrinkle in the wrist seal (which is trimmed down in my case) the whole time... Which really was the core cause of the leak.
I'm amazed at how resilient the system is that I was able to dive for so long with a bad seal on the suit - and not have a leak.
...So no, I don't think you'll have a problem with leaks.
Just to be sure - your suit came originally equipped with SiTech QCS Oval rings, right? Not the round ones? You pulled them (and the wrist seal) off and replaced them with the Waterproof Ultima Dryglove Ring system, right? With a wrist seal between the suit's soft ring and the Waterproof suitside hard ring (that the glove clips to)? The system should fit snug, but not hard at all. I always notice a little "snap" sound when I have installed mine. Did you hear the "snap?" It's more prevalent with silicone seals than with latex seals... But present with both.
I, too, have gotten pretty good at installing the 720s on the rings. It takes me just a minute or two, and without tools at all. Since getting the technique polished, I've been planning on making a video - but have been too busy to put it together. I'll do that today and post it so that y'all can see what seems to be working here.
Yes, absolutely... The thinner and stretchier 720s are WAY easier to install on the rings than the 660s. Huge difference - and frankly, a big reason to be a fan of the 720s right there.