Firmware on the machine = firmware on the printer, not the PC. I normally associate machine with a PC too.
Correct, there does not appear to be a new firmware released for this printer model on any OS right now.
You mentioned trying both PCL and the printer specific print drivers already. Try the post script driver and see if that has any better results.
Normally when there is slow network performance while printing the print driver is the problem. Specifically the print driver has a hard time converting the document into printer language which causes extra processing and stalls the job in the local queue. The easiest work around is to try a different driver until you find one that performs to your satisfaction. The Win7x86 computers would be using the same network as the x64, the only piece that is different is the print driver installed on those PCs.