I couldn't be less concerned about the roll over risk for a couple reasons:
1. The modification of 2 1/8" and 1 1/16" hole will make no real world difference in regards to the ROPS ability to maintain structural integrity. Even if it did, I weigh less then the average American and that 80lb difference more than offsets the 3/8" of metal removed
I'm no structural engineer but that fuzzy math makes sense to me.
2. If I roll it over....I ROLL IT OVER. It's not Kubota's fault that I put the machine in that position. They built it to be run wheels down and it's my job to ensure I operate it correctly. The sue happy world we live in has pushed common sense so far away people would rather live with something like a easily corrected noise issue than loose their right to sue over an accident.
I've scoured the interwebs and been unable to find a single case where a modified ROPS has failed. While that doesn't mean it couldn't happen it's a pretty good sign. I had no problem finding people who have won the lottery...I like the odds on the ROPS mod.