If you're gonna run converters, you need to use the smog pump.
The air(from the smog pump) helps break down the excess HC (hydrocarbons) and CO (carbon monoxide). The air supplied to the catalytic converters helps create the catalytic reaction that changes the HC & CO into CO2 and water vapor. Catalytic converters on 5.0 Mustangs are designed to use the extra air provided by the smog pump. Without the extra air, the catalytic converters will clog and fail.If the h/xpipe has high flow converters and no pipe fitting welded near the converters(for the air injection pipe to connect to)you don't have to run the smog pump.If the h/xpipe does have this pipe fitting, you should use the smog pump.The smog pump only uses 2-4 HP during wot,so its not too bad in the parasitic drag dept.The xpipe will give your Stang a higher pitched,raspy sound vs an hpipe.The hpipe gives that deeper sound.If you're gonna run cats and the fitting is there already, I would leave the smog pump in place.
Oil consumption and poor tune increase the amount of HC the oxidation catalyst has to deal with. The excess HC, that the converters cannot oxidize due to a lack of extra air (caused by deleting or bypassing the smog pump)becomes a crusty coating inside the honeycomb structure. This effectively reduces the size of the honeycomb passageways and builds up thicker over time and mileage. Continuous usage under such conditions will cause the converters to fail and clog. The extra air, provided by the smog pump,is essential for the oxidation process. It oxidizes the added HC from oil consumption and poor tune and keeps the HC levels within acceptable limits.
Newer catalytic converters do not use the Thermactor Air System (smog pump) because they are designed to work with an improved computer system that runs leaner and cleaner,but this type also uses 2 extra o2 sensors,which the 86-95 Mustang doesn't have.If you decide to run a vatted h/xpipe,install it first to see how the car sounds & if its still too loud or the sound doesn't satisfy you,go ahead & add whichever muffler meets your satisfaction.
D.S.S. 331 10.5:1
Trick Flow Top End
80mm maf/70mm tb
1 5/8" Longtubes
Cat Hpipe/2 Chamber FM