Nah Dan's right. I think the biggest problem with the O.E. one is because they look crap people assume they won't work as well as a big shiney front mount. Like Dan says G40's don't have the high compressor temps associated with most forced induction engines.
My old G40 produced 181 bhp on Aldon's rollers with the standard intercooler (it was the original one and had seen 168k of use so wasn't in the best of condition either). Whilst it obviously wasn't the the ultimate setup it didn't stop the engine producing the goods. So I would say the O.E. intercooler will be fine for all but the most highly tuned race G40 motor's.
However, fit a turbo or Eaton and a decent front mount intercooler is a must if you want respectable power.
Well said! If any recalls some inlet temps I did back on the old PSD forum, I was achieveing 15PSI on a 51mm THB, GT manifold, runing 65 pully whilst boosting at atmospheric temperature(at throthle body inlet) add some water methanol to it and I did actually start pushing temperatures below atmospheric(impressive, but also had to contened with icing on the THB), anyway, all this achieve on a standard(and dare I say it cos it is a 'front mounted')intercooler.
Biggest problem is that many older cars have filthy dirty OE intercoolers, take it off give it a dam good clean, straighten up any damaged or bent fins, refit and feel the improvement, and if you comfortable with your charger condition and service histroy remove the gauze on the intercooler inlet. Hey presto you got something you don't need to replace unless your seeking 200hp(cos things start getting a bit hotter then).