I've done a bit of drying out on stock TS and SS style exhausts. The chamber on those gets the water just under the rubber coupler. You should see a rubber hose running from there to the larger chamber section. Pull it loose and slip a restrictor inside that hose. Those little 1/4" thick round spacers that come with a new battery for the bolts work great and won't overheat anything. I've not tried going any smaller.
The one that I cut open was a completely dry chamber stock. I can't remember if I cut open a 750 pipe or a 650 pipe, but I would be surprised if they are not both dry chambers as the two systems are very similiar. The water from the water jacketed section comes out around the outside of the pipe under the rubber boot into the waterbox. I have an extra line out of my head to bypass some of the water out of the waterbox, but it's not the same as the "dry pipe" modification people do to the 650sx pipe which has a ton of water running through the chamber in stock configuration.