Are you speaking of tuning plate surface temps, cook chamber temps, or cooking grate temps?
On your tuning plate, did you leave a gap of about 2" on the vent end?
I found that it has to be able to flow across relatively freely to get even temps all the way through. With holes drilled in the tuning plate, it only allows a small percentage of convection (heated air/gases) to pass upwards through the plate, the rest of the heat is radiant (through the plate metal).
I have my tuning plate fitted level and tight on the firebox end and front-to-back. I use long stem turkey fryer therms with the tips resting directly on the tuning plate. These were installed through small holes drilled just below cook grate level in the front of the cook chamber. I can monitor the tuning plate temp very well. The fire box end is always 50-70* hotter than the vent end with a gas burner, and about 30* hotter with a charcoal fire with mine.
As for the chamber and grate temps, have you checked both ends for even readings?
Grate temps are what you really need to know. Chamber therms are a baseline to go by after checking grate temps and comparing with the chamber therms, so you know what the difference in readings are.
Depending on what you are seeing for temps, you can go alittle further with the tuning plate by drilling a few more small holes in strategic locations where ever the cooler spots are, to even out the grate temps more. Don't drill too many at once, or it can foul up the process. It took me a long time to get mine relatively close, ~10-15* front-to-back, and ~15-20* left-to-right. Trust me, this takes time, patience and lots of oven rack therms to save some time (I have 10). 20-30 minutes between each temp check with using a chamber therm for a baseline.
There is a good side to having uneven grate temps though...if you have birds to smoke you can put them on your hot spot and run the butts/briskets, etc. on the cooler areas for slower cooking.
I tossed out what I could think of based on what you gave for info so far...come on back if you need more/better info.