I ran a capacitor discharge ignition system in my 65 Rambler American flat head 6 with a really big 0.055+ spark gap and would see 2 inch sparks jumping from the plug tops to the head on damp days. It allowed me to lean out the a/f mix pretty lean which you can't do these days so I don't think you will gain much but it will take care of those times when it is hard to ignite the mix like when cold starting and when you hit the gas pedal fast.
you won't see nearly as much on something well designed as a honda as you will on as old of a design as a flathead. I don't think it'd pay off in the long run for many years. you'd be looking at like .1mpg difference unless there's something malfunctioning in your current system. if you have to replace the system anyway, may as well upgrade.
1991 Toyota Pickup 22R-E 2.4 I4/5 speed
1990 Toyota Cressida 7M-GE 3.0 I6/5-speed manual
mechanic, carpenter, stagehand, rigger, and know-it-all smartass
"You don't get to judge me for how I fix what you break"
I'm currently running a .52 gap versus .44 (~20% increase) on my Acura Vigor. It helped my cold idle, low rpm torque (can shift at a lower rpm now), and mileage on E85. No other mods to the ignition. I may get the MSD-5/5200 eventually. It provides 3 sparks per combustion cycle at <3000rpm. Should improve low rpm combustion, especially if you're running lean at all. For E85 users like myself it will really help out with cold starting and low rpm running before the engine warms up.
Bigger gap means more electrical stress on the insulation so you have to be careful how big a gap you use. Other than that adding more spark energy ensures that the fuel is ignited and more completely burned which is a good thing. Unless you have an ignition problem and most cars do at some time or other under some non-typical operating conditions then you may not see a big improvement but you should see some improvement.
On my explorer we installed a much stronger ignition coil similar to msd, and increased the gap to .065. The bigger coil is able to jump the gap without misfiring. It runs so smooth that I constantly think it isn't running at all and have to watch the tachometer. The throttle response is immediate, smooth, peppy. The mileage is probably 1 mpg better.