Well, if you''ve never replaced it, it would be simple as looking at your module and seeing what the Motorcraft part number is. I've never heard of different TFI modules giving you more "low end." A TFI module's job is just to switch current flowing to the coil (or from, if you believe in electron flow theory
), to interpret the signal generated by the hall effect switch, and execute the timing signal from the computer. Theoretically, a longer dwell might give you a hotter/longer duration spark, but on a stock application (or most high performance for that matter) it won't matter all that much at idle because cylinder pressure is at a minimum already.
If I were you, I'd just have the service done, if it's for free and your Mustang qualifies for it. If it doesn't, I'd just replace it with a quality aftermarket module, put some new plug wires on there, and be done with it. The reason I say plug wires is because about 90% of the time, when ignition modules fail, it's related to old or broken wires. The is because all of the current that's supposed to go through the secondary circuit (which kicks the 12V up to 50kV) has no where to go and ends up as a nasty voltage spike at the module. Those tiny transistors won't take all that abuse for too long, hence they stop working when they get hot. Be sure to put a good coating of
heat sink compound, NOT DIELECTRIC GREASE on the back of the module before reinstalling, and it should last you for the life of the vehicle...at least that's been my experience. Hope this helps.
p.s. If I remember correctly, Ford's fix is just to relocate the module to the fender, as they did on later TFI setups (started with '94 Mustangs). They finally got smart and started mounting them on GIANT heat sinks.