So what would be the drawback of fitting the 2.5 gearing
Economy mostly. Keeping the revs low during cruising cuts down the losses in the engine. Might take a bit off the maximum speed too I suppose.
If the engine's got the grunt, it's much better to cruise with low RPM and a wider throttle opening than at a higher speed where it could potentially generate much more power than required but is being throttled back.
Production cars are about a compromise of all factors, not just fastest to 60 etc. after all.
In fact, my main moan about most manual cars is that first gear is designed to pull away with a caravan on the back, a bootful of luggage and a back seat full of kids, and a less than competent driver at the wheel, without burning the clutch out. Conditions that my car rarely ever sees (ok, maybe the incompetent driver...). I'd say the auto has the upper hand here, because first in a manual will just light up the back tyres when given full bore in first without a full load, whereas in an auto the combination of a higher first and a torque converter that will give a lower ratio for just as long as it takes to get the engine up into the torque means that the initial 30 feet might be slower, but you won't be reaching for another gear at 35 MPH.
So, replace first and second gear with one gear almost as high as second but not quite, and the aforementioned 4 speed Omega would probably do pretty much as well as a 5 speed. Wouldn't be fun with a caravan though.
Kevin