Re: TC30 w/ 84\" RFM
Well I'm the guy (or perhaps one of them) who often tosses out the 3 to 5 PTO hp for every foot of mower deck number. I do it for good reason, that is each of us has different conditions, but the numbers stand up pretty well as baseline numbers.
I can tell you that my 18.5pto hp tractor with a 60" deck has absolutely NO problems under 95% of the conditions. However, when the grass gets TOO LONG and/or is wet (I really hate mowing so if the lovely Mrs_Bob slacks off in her mowing duties -she has her own tractor- then I get out to do some mowing) anyway, if it is long or wet I can begin choking my tractor in the lush spots of the lawn. Now I also have some reclaimed fields that I've never had a problem with mowing because the lawn is sparce.
Bear in mind, I am one of those belt & suspender guys. I would rather buy a slightly smaller implement and have all the HP that is needed when conditions are tough, than buy a slightly larger implement and struggle when I can't get out to mow every single week. I see folks recommending 5' rotary cutters on small tractors and then saying just cut it 1/2 width when things get tough, or recommending a tiller that is 12" too wide and then justifying by advising that you just have to make 2 passes to get to full depth. Both of those types of things simply mean the implement is over sized.
Now all that said, many folks probably can use an 84" mower on a TC30 if their conditions are right. If the grass is kept mowed. If the lawn is generally sparce and the climate often arrid, then I would suggest you don't have a real big issue to worry about. But if you have a think lush lawn, or a nice fast growing rye or bluegrass lawn, or regular summer rains that keep the lawn growing, or you just don't like to mow very often . . . so please take the 3-to-5 PTO hp as a guideline, and then consider your own conditions. You might be fine or you might not. But many of us sitting on our side of our computer screens can't really see what is on your side of your computer screen. I'd rather give out a broad recommendation than a specific if I don't know what your exact conditions are.