Altitude always makes a difference on an internal combustion engine. The density of the air will dictate how much boost you put into a setup. The good thing about forced induction is that you can compensate for the density of the air at higher altitude. That's all a supercharger or turbo does, it increases the density of the air. If you use the ideal gas law, PV=nRT. n and R are constants, the only thing that changes will be pressure, volume and temperature. Consider volume constant in this case, and you're left with pressure and temperature. The initial pressure is that of the atmosphere where you are, and the same for temp. The final pressure will be atmosphere plus whatever your boost pressure is. You solve for temp, that will be the intake air temp the engine sees. Naturally, the lower in elevation you are, the less work the supercharger will have to do to reach the same pressure, lowering your IAT's. The more you compress air, the hotter that air will be. With an old school blower, you don't have an intercooler so you'd have to inject something cold to bring down temps, be that methanol, water or nitrous. You could theoretically create a custom blower plenum that has an intercooler, not sure if it has ever been done for the FE.
If the temp is 75 *F outside, at sea level atmospheric pressure is 14.7 psi, 4000 ft the atmospheric pressure is ~12.75 psi and at 6000 ft atmospheric pressure is ~11.86 psi. So in order to compensate for elevation at 4000 ft, the boost would have to increase ~1.95 psi and at 6000 ft, the boost would have to increase ~2.84 psi. When you consider the fact that you monitor boost pressure by at least 1 psi if not smaller, that is important. At 4000 ft, you've lost 13% of your initial pressure and at 6000 ft, 19%. The temperature outside plays a huge role as well, the colder it is the more dense your air will be as well as having colder IAT's.
I should note that I haven't discussed how the fuel will impact all this. The more fuel you inject, the colder the incoming charge will be. If you were to try and create an intercooler for an old school blower setup, you'd be restricted to either mechanical or electrical fuel injection at the individual runners. The intercooler would trap too much fuel, which would be dangerous besides making it hard to tune.
