I have the D.UC bin file that I copied from my 4824P before flashing it to a 5224P. I had already flashed it from D.LC to D.UC before I backed up the firmware. If you want to try it, send me an email and I'll send it to you. I just burned a 603MB ISO in 2:32@48X w/ drive flashed to 5224P using the K.RC bin(cptsnoopy hooked me up!), then flashed to K.UC. I don't have any 52X media yet, but I imagine it should burn @52X. Sorry to hear your having problems, have you tried flashing to K.UC and seeing if it still only burns @12X?
It's all in the reflexes..