Three things...
1. The use of peltiers in this configuration will be VERY ineffective. Especially when on the same power supply.
2. The plumbing arrangement is questionable at best. Unless there are two outlets and one inlet on the reservior, it will only succeed in doing what you say it's doing as the CPU block is in a split loop with part of it returning to the reservior.
3. The use of aluminum as opposed to copper for those blocks is not going to give you the performance you need with peltiers in that arrangement.
I would suggest replumbing it so that the pump outlet ( I presume there is only one with the black ring around it at the reservior ) goes to the CPU first, then the GPU, then loops through all the peltiers THEN back to the reservior. Then cap off the second return.
If you don't see a big enough improvement with that, put each of the peltiers on thier own power supply.
My 2ยข worth...
