Gasser,
I have the same tractor with the same mower deck (new this past spring). This fall I hooked up my old lawn-vac system that was previously adapted for my JD445, and it works even better than it did on the 445. My system is a older brand lawn-vac with a 6hp engine sucking leaves through a 7" ID flexible hose from the deck and then through a 6" hose to the trailer. The engine mounts very nicely on the tractor with the trailer tagging along behind. I bought the JD three point hitch adapter that allows a reese-type trailer hitch/receiver to be inserted and then welded up a new mount for the vac engine mounted to a piece of 2" x 2" square steel tube. The tube inserts into the reese receiver and locks in using a pin. I needed to buy a longer 7" hose to account for the additional length to the motor and a new hose-to-deck plennum. By the way, have your Deere dealer order the plennum from Trac-Vac, which is a tag-along lawn vac system Deere distributes. The plennum is custom fit for your deck and comes on and off in 15 seconds with one hinge pin. Easy to hinge up to clear if clogged as well.
All that said, the 6 hp engine easily keeps up with the mower deck with the tractor RPM's at the recommended ~2900-3000 rpm. I wouldn't reduce engine speed to slow debris to the vac. I just don't think it's good for the machine and the vac will easily keep up with the deck. I never run the vac engine at full speed - no need. If I did I would suck all the moles out of their dens... hmmm... not a bad thing now that I think of it!
The plennum is built to match your hose diameter. My vac originally had a 6" hose to the deck but I increased the size when my 445 plannum came arrived with the larger diameter fitting. Had a new fiiing welded onto the face of the vac fan shroud and it works just fine.
One thing to note, as the fall moves on the grass is no longer growing, lower the deck one or two notches from what it had been earlier in the fall. The vaccuum effect around the lip of the mower deck is significantly increased and it will suck up everything rather than blow some of the smaller, lighter leaves out to the side. Found that out this past weekend.
Enjoy and good luck. If you know anyone with a 54" deck, I have the plennum and new 7" hose that I can't use. Case of good beer and it's theirs!
Mark