Jim -
I put @ 32,000 miles on a FMS 392/475(?)hp. Replaced it with a Roush 427R/550hp which now has about 14,000 miles on it. With the FMS I was constantly doing upgrades trying to get a little more power. Pro Systems carb, Trick Flow heads... With the Roush I had an AED carb custom built and dynoed for the motor to replace the POS vacuum advance 870 that Roush shipped on the engine. I made that change as soon as I got the motor because I hated the vaccum secondary throttle response. I have not felt the need to make a single change to this motor since then. I have felt the need to put BIGGER BRAKES on the car!!!!
There is a huge difference between the 392 and the 427, both seat of the pants and measurable. The 392 as it came from Ford would run low 12 sec 1/4 mile.
After all of the upgrades I did to it I was running 11:70s. The 427 currently is running 11:30s. I think that when I get my launch perfected I will get into the 11:20s. In short, there is a HUGE difference between the FMS392 and a Roush427R. Hope this helps. Let me know if you have any specific questions.