Keith,
I have wrestled with this question a lot. Were this me, I would tighten them another one to one and a half turns. I think a loose mount gives the engine and cradle a chance to rattle around a within its mountings. It seems to me that keeping the ISO mounts tight, yet not so tight that it inhibits the ability of the rubber mounts to absorb shock, it will keep vibration to a minimum. Be sure that when you tighten the mounting bolts that the bolts do not get close enough to the engine to pierce the engine cowlings or casting. It is possible that one and a half turns could be too much.
I had a terrible vibration problem last summer with my 1250 Nightmare. When I got to looking, I found the ISO mount bolts loose in the frame as well. After tightening them, and running the machine, and tightening them, and running the machine, then loosening them and running the machine, I found a sweet spot of the least amount of vibration. I added a couple of pieces of felt where the hood meets the lower grill casting, just next to the hinge, and I can finally trust that the fillings in my teeth will not come out.
This isn't scientific, but it worked for me. It took a lot of trial and error, but was worth every bit of it. If I can help any further, please let me know.