It's impossible for a 7200RPM drive to slow down to 5400RPM without completely FUBARing the drive in every way humanly possible. It doesn't matter if you used a 300YD IDE cable with power electromagnets wrapped around it, the 7200 RPM IDE drive would still continue to spin at 7200 RPM, although you'd get *0* bytes of throughput.
Remember to try to keep the drive with the OS and everything else that is accessed often on its own channel. Storage drives, burners, ZIPs, etc. Should go on the secondary channel or a PCI card IDE add-on controller. Unless you're in an extreme situation (like me), don't put burners on the same channel with the HDD you're burning from.