Personally, I think it is a pretty decent and inexpensive route to go. There are Supra guys running 800-900whp with the MAP ecu, so it definitely works. IMO though, the Greddy E-Manage Ultimate
is the best piggyback type device like this on the market. Both of these units basically perform a large majority of things that a standalone would. Not to mention, the hardware of both of these units (cpu's, circuitry, etc) is superior compared to the relatively old 3.2 Carrera Motronic. There's really no way you could go wrong, as long as you know how to tune or know a good tuner. Unfortunately things like MAP and MAF conversions for Porsches tend to be very expensive. In all honesty, units like the Greddy E-manage Ultimate are probably more advanced then any of the map/maf conversions out there for our cars, not to mention it's cheaper. As technology advances, things tend to become cheaper so there really is no need to pay $2-3k for maf/map conversions any more. True, a large part of what you pay for is the tuning and knowledge behind making these things work. Though, we're not in the 80's anymore!
With all of the info on the internet, the vast amount of competant tuners out there, etc there are now plenty of viable and inexpensive options like these piggyback units. Hanging out with the import crowd much of the time, I've seen how well piggybacks can work and how effective they can be.