What did the old coolant look like when it came out?
If it was still 'clear', then you might still be ok.
I've never heard of the impeller blades wearing out, but stranger things have happened before with these machines!
Are you sure you got all the air bubbles/pockets out of the system?
When I do my coolant, I run it with the res cap off for awhile, turn the heater to full hot, but leave the blower off, so it keeps more heat in the engine.
Rev it up, let it idle, rev it up and repeat for about as long as it takes to slowly down a beer, then I put the cap on and take it out for a spin! (as long as I haven't had too many beers while doing the flush! ha)
It's worked for me so far.
Do you have an aftermarket water temp gauge? Or do you know for sure you are overheating?