Matt, you have a lot more experience with Cubs and the amount of vibration one should expect than I do (actually, all your experience vs my zero
) But here's my take:
The QL was designed to compete with JD and others on quiet exhaust and low vibration. The previous generation of wideframes had solid mounts. In the QL series, they put in the rubber mounts and, let us not forget, the notorious balancing gears.
Later, IH changed the rubbers to a firmer version, acknowledging the weakness of the early soft mounts.
So if you stiffen your mounts using a different (likely more durable) rubber, maybe you lose a little of the original Quietline smoothness. But I wouldn't lose a lot of sleep over it.
I'm putting the $20 set of ISOs on my 1450 while it hosts the 1650's K341 over the next several months. It'll give me a chance to try them out and see how they look after some usage.