Hi Benski,
Welcome to the site! There are a lot of great folks here with good advice when you are looking to solve a problem.
I fully agree with pblanc on the stability issues with the Perception saddle's top seat. I have had 2 of them that came in canoes that I have bought and I pulled and sold both of them. I did try out the Perception in both canoes, but a good setup with foam saddle, thigh straps, knee blocks, ankle blocks and toe braces is the way to go. I especially recommend setting up some ankle blocks, a lot of paddlers overlook them but they give a lot of support that comes in handy after an hour or so. It also doesn't hurt to get out of the boat and stretch when you can. Don't forget, Ibuprofen is a C-boaters friend.
What type WW canoe did you get? I recently picked up an Old Town H2Pro and outfitted it although I have not dropped it in whitewater yet. I'm looking forward to seeing how this 30 year old boat paddles.