Depends on the choice of blocks and pump. Performance curve of thick baseplate wbs (#rotor style, MCW5000A, AquaJoe, etc)is pretty flat; I'd think even adding fairly restrictive GPU and NB blocks would not affect CPU temp more than 1-2C as long as your pump can push 1GPM through it. That's the beauty of the Swiftech kits: small ID tubing for easy routing & the ability to add multiple blocks wihtout compromising CPU cooler performance.
With thin bp blocks like Dangerden RBX, DTek WW, and LR Cascade, there can be a much larger effect on performance when you lower flow rate. If you have a low power pump or 3/8" tubing and are using such a block with restrictive GPU and NB block then easily 4-5C difference in CPU temps. Choice in pump, and extra wbs becomes pretty important here! Best bet is to go with low resistance GPU and NB blocks. Going by JoeC's reviews (at 1GPM) Dangerden Maze4 chipset block looks to be an excellent choice for the NB; whereas either Swiftech MC50 or DDen Maze4 GPU work well for GPU.
Question would be "does 4-5C matter to you?" If so, then may (will) be cheaper possibly to upgrade CPU wb or pump than to pay for replacement nb and gpu blocks. Might be that you are getting enough benefit (quieter operation and lower GPU/nb temperatures) from the current setup to outweigh 4-5C anyway.
I am fairly obsessive and so perhaps not the best person to ask
