I've tried a few 4 and 5 speeds. Here are my thoughts, for whatever they're worth.
AWY: Is a piece of *** trans (3.94 R&P and a .89 5th). Even if you swap in a .71 5th gear, the transition from 4th to 5th might make it useless on the highway, unless you've got a TD. I only say this in the instance that you drive on a lot of hills and/or drive in mixed traffic. Shifting from 4th to 5th is a large change, and getting up to speed in 5th may take some time.
FF, FO, FN: These are a nice fit for the 1.6NA (3.89 R&P and .71 or .75 5th). Unfotunately (as some note), they won't fit into a MK2 or later. The gearing on these is really well-suited for both 1.6 NA and TD applications.
4K, 2H; These came on the 83-84 GTi (3.94 R&P and a .89 5th). Yeah, I was dumb enough to try it. HORRIBLE idea. This trans is even worse than the AWY if that's possible. You can't go over 55 in 5th. This means, 48 in 4th is about your max in terms of RPMs.
7A: If you can find one (RARE), this is the best way to go. 3.94 R&P, and .71 5th. A rare score. The best of both worlds. It feels like overdrive in all gears after 2nd gear. This was VW's "4+E" trans that came in the late 1983-84 Rabbit and Jetta (both gas and diesel). So, if you find a stock diesel from those years, check the code. Also, I've pulled a few 7A tranny's from 83-84 Jetta gassers.
AVX: A bit over-rated in my mind. the 4.25 R&P means quick acceleration. But the .75 5th means the engine is comfortable at about 65-68 MPH. If you can swap in a .71 5th, this might be a great setup, but still second fiddle to the 7A.
AOP, AGS, ACH: These came stock on the 1985-1989 Jetts/Golf in the US (3.94 R&P and a .75 5th). They are a very nice complement to the 1.6 NA. Absolutely no complaints here. Obviously, a .71 5th would make them better, and effectively, they'd be just like a 7A.
ASF, ACN: Nice until you have to accelerate in 1st, or backup. The R&P is so low (3.67), the car loses torque. It's a good fit for 1.6TD and 1.9 TD motors, but not so much any 1.6 NA. The top speed is great, but again, it's similar to the 4-speeds (see below) in that the torque is absent at higher RPMs. These transmissions are only recommended if you spend 80% or more of your time on the highway and/or you've got a TD.
4-Speeds: There are a few. I haven't memorized the codes, because they all pretty much suck, and it's not worth it. But, if it's got a 3.89 or 3.91 R&P and a .71 4th gear, then yes, they're great on the highway, provided you're not climbing any hills, or have to accelerate up one. If you have one of these trans and are considering a 5-speed swap: simply wait until you find a good deal on the right trans, since these are pretty darn good and not worth swaping out unless you get a REALLY good deal on an FF,FO, FN, 7A, AOP, AGS, ACH, etc, or your motor is really tired and can't push enough torque to acclerate to 65-80 on the highway.
Any 4-speed that came stock on a Rabbit Diesel Pickup (not all the codes appear in the sciroccor.org/gears page) is gonna suck, so don't waste your time. Nearly all had 3.91 or 3.89 Ring and Pinions, and .89 4th gear. This was done for the 'power' aspect that trucks should have. Acceleration to 60 MPH is nice, but you're screamin at 70 MPH.
I guess the bottom line on the 4-speed is: nice at top speed in 4th, but getting there can blow (unless your 4-speed trans is from a Pickup). If your 1.6NA is still in good shape (compression and IP/injectors) and you can push it, then leave it alone and keep the 4-speed.
Bottom line: not all 5-speed swaps are in your best interests. For your application (original Poster), I'd stay with the FF. Down the road, if you find you want to lower the RPMs, then go with an ACN or and ASF (dime a dozen at most Pick-Your-Part J-yards). With a TD, the 4-speed will pretty much blow, and can't compete with the .71 5th and 3.89 R&P.
Just my $0.02
