First off, those of you that have encountered the dreaded fuel cut know that more fuel is needed at low to mid RPM. The factory pump is not capable of building the requested amount of fuel pressure necessary to feed increased boost at low RPM. Many of the tuners have masked this by starting the line pressure higher ( raised from ~110bar to ~130bar) before the engine/ turbo make their way up to fuel cut. Sometimes this allows the engine to accelerate past the known fuel cut scenarios before the fuel pressure drops low enough to trigger a fuel cut. As chip tuners get more aggressive with their fuel and boost maps near boost onset, this problem gets worse.
By increasing the volume of fuel displaced each cycle the pump makes (3 per 720 crank degrees), the pump is capable of pumping more fuel for a given RPM. For example, lets say that Brand X pump is capable of pumping 100cc/min. of fuel @ 3000RPM, and 200cc./min @ 6000RPM. The engine may only require 110cc. of fuel @ 3000, and 180cc @ 6000RPM. In the above situation, it's the lack of fuel volume @ 3000RPM that is the limit, because at 6000RPM the pump is capable of flowing 11% more fuel than needed. Let's say that Brand Y increased the displacement of the pump, and it's flow capacity goes up to 135cc/min @ 3000RPM, and 270cc. @ 6000RPM. That means that the upon installation of Brand Y's pump, the engine will be recieving all 110cc/min of fuel it requires, along with an extra ~23% excess. Now let's say the ECU is flashed with software that ups the boost in the midrange and tapers it down near stock levels @ 6000RPM. It is concievable that the engine would now require 130cc./min @ 3000RPM, and 190cc./min @ 6000RPM. With Brand X pump, the engine would definitely hit a fuel cut @ 3000RPM, but the pump would be capable of keeping up @ 6000RPM. Add Brand Y's fuel pump and the engine can take in all the fuel it requires, at 3000RPM, and 6000RPM. Now the dilema lies in increasing the volume of the pump enough to meet demands but not so much that the ECU controlled regulator/ bypass valve is outflowed.
Some pumps ( or kits) that will be coming to market have seals that are OEM, and some are high quality aftermarket seals. Both are acceptable. The tolerances inside the pumps are very tight, and care must be taken by whomever assembles the pump. Some pumps will be assembled by the company, some will come in kit form requiring (sp?) assembly. It is not a dificult task to assemble a pump. Assembly can be done with one socket, a lint free towel, and clean hands.(overly simplified, but after all, that's essentially all that is needed) For those that are not capable of, or afraid to assemble their pumps, I am sure the company manufacturing the pump will perform the assembly using your pump housing for a small charge.
Now on to the myth about quality vs. cost... Just because a product is less expensive, doesn't mean that it is not lower quality. Sometimes a company may be able to hold a tighter tolerance, and still charge less. Especially if the parts are "easy" for them to deal with. For example, the company that "fits" our pump parts, deals with tolerances 5 times more accurate than we require on these parts. To these guys, 25 millionths of an inch is a clearance (and 5 millionths tolerance) on some parts. So the fact that we have a tolerance of that same 25 millionths makes our parts "easy" for them. These same guys deal with parts very similar to ours every day of the year for the aerospace industry. I am sure that most of, or all the pumps that come to market will have no more clearance than the .0002" that the factory parts have. Some companies measure in metric, so let me tell you that 1 Micron is equal to a little over 39.37 millionths of an inch. So in essence (sp?) our tolerance is ~.635 Microns.
In short, the fuel pump won't increase the peak hp. capacity of stock turbo cars, but it will allow more aggressive tunes at lower RPM.
I think that's a pretty good start towards an education on the new FSI rail pumps that will be coming to market soon.