Re: diesel engine, no way is it slow. Especially at a mile above sea level. Up here in Johannesburg, the diesel almost matches the V8 on performance because of the turbo. Fuel economy is also far superior, eg. 13l/100km vs 20l+/100km for the V8. At sea level maybe a different story.
Different strokes for different folks. You need to drive the new TDV6 to believe it. Very quiet, very smooth, and powerful. My previous disco was the TD5 (also Diesel) and we had years of happy motoring with it, despite that because of the noise the neighbours thought we owned a tractor...
BTW, locally the main expressed use for the snorkel is as Tim says - dust, especially when travelling on convoy, the snorkel will allow your vehicle to breathe cleaner air above the level of the dust thrown up by the vehicle in front of you. At least this is the plan

But I think with the side-mounted intake it may be unneccessary.
If you are gonna cross water deeper than the wading depth, you also need to make sure all the other systems in the vehicle are equipped with breathers that exit far above this level, and this entails electrical systems, lights, battery, gearboxes, diffs, etc. etc.