As some of you already know, Porsche introduced the normally aspirated 993 in 1994 with a relatively close-ratio gearbox called the G50/21 (G64/21 for AWD). However, cars bound for the United States, Austria and Switzerland, had to have the G50 (G64)/20 gearbox with taller and slightly more spaced gears to meet fuel economy and noise restrictions in those countries. As of MY 1997, Porsche installed only the /20 gearbox on all normally aspirated 993's worldwide.

Recently, I had a G64/21 installed in my '96 C4S. The gears from 2nd to 6th are all shorter and 2nd is slightly closer to 1st while the gaps between all other gears are also slightly smaller. This combination of ratios allows much better acceleration throughout the rpm range of the engine.  Previously, I drove around town in 3rd and barely used 4th. Now, I find myself mostly in 4th and 5th while on the freeway 6th can finally be used at legal speeds without lugging the engine. I never used 6th before because it did not accelerate at all from just about any rpm. With the shorter 6th, one can stomp on the gas from 2200 rpm and the car goes.
On the track and in tight turns, I always looked for the "right" gear between 2nd and 3rd the former being too short and the latter too tall. I expect  the new shorter 3rd will be more adequate in such situations. The 1.024 fifth will also be very useful on long straights coming out of turns such as  at Willow Springs.

During this gearbox transplant, it was discovered that the dual-mass flywheel was starting to go bad. This flywheel is not too reliable and can fail  early in some cars. I decided to replace the overweight and unreliable flywheel with a lightweight RS version. This also necessitates the change in clutch and pressure plate. The new flywheel does indeed allow the engine to rev quicker. It takes less time for the engine to reach the more  usable power band and the "Varioram" lag I experienced before is much less noticeable. I would estimate the speed at which the revs climb is about 20-25% quicker than before. On my '96, there are no stalling issues and the noise from this flywheel is a muffled "brrr" only perceptible at part throttle. Once the car is under way, it sounds exactly the same as before.

To sum-up, these upgrades are substantial and worthwhile. At least, to me they were. I think any normally aspirated 993 can be brought to life with at least the close-ratio gearbox. The lightweight flywheel is a welcome bonus that might complete the package for many. I was also very pleased by the shop that did the work. E-mail me if you'd like a recommendation.
 

                             /21:

                             1st - 3.818
                             2nd - 2.150
                             3rd - 1.560
                             4th - 1.242
                             5th - 1.024
                             6th - 0.820

                             /51 (worldwide):

                             1st - 3.818
                             2nd - 2.150
                             3rd - 1.560
                             4th - 1.212
                             5th - 0.937
                             6th - 0.750

                             /20 (US gearbox):

                             1st - 3.818
                             2nd - 2.048
                             3rd - 1.407
                             4th - 1.118
                             5th - 0.928
                             6th - 0.775

As you can see, 1st through 3rd are the same compared to the twin turbo while 4th is very close. I suspect that by the time you've shifted into 5th or 6th you're already running at stratospheric speeds (read boost) in your Turbo. However, you can always custom tailor the gears on any car to suit your driving style and needs. In the case of the normally aspirated 993's with the /20, the car's downright "need" the closer ratio gearbox.

As to the lightweight flywheel, the benefits are similar in any 993 variation. There are several 993 Turbo's running with lighter weight flywheels.
Also, I would imagine the DME would take care of all stalling issues as it does on '96-on normally aspirated 993's.
(Answer by Viken)