I have a VX 8100 and I've put on a ringtone of over 700K and another over 500K (using bluetooth and bitpim). Both are the same quality, size, etc as the source, even after using them. I copied them back over to my computer and did hashes and other comparisions, the file is exactly the same. Also, the quality is amazing on the phone. When I copied the files over to the phone, I had to delete a certain index file from some other location, then restart the phone.
My friend just got a VX 8300 and we put all of his old ringtones on, which included midi and amr. After copying the files over, he restarted the phone, and all were available. No files to remove. None of them were over 100K, but the sound quality was the same as on the computer.
I highly recommend using either bluetooth or a cable. Why waste money/time/bandwidth downloading them via PIX?