Me has Twintercooler too, and the S3 cooler! 
....Did you install them both in the same session? (seems you didn't from your 2-day posted info) In other words, did you install the S3 i/c first, run with it awhile, and then install the Twintercooler later? (Or vica-versa of course) and consequently form the opinion that the S3 i/c (alone) was of significant benefit.
Cheers.
Both together.
The Forge alone is a piece of wee-wee to fit. Ten or so torx screws to remove the front bumper, a slight mod to the horn mounts, and a few other minor bits and bobs. If you were reasonably proficient, with the appropriate tools (and downloaded the appropriate maunals from my sticky), then the complete Forge-only install could be done in about 90minutes. Look on the Forge website, and download their very comprehensive fitting instructions.
The reason I took so long, was simply because I was working alone, taking lots of pics, making 3 or 4 pages of notes (for when I get round to doing a "how to" guide
). I did have issues with the hoses, which still are not fully rectified (hence my thread about Forge c/s in another section of the forum). Technically, the whole job lasted 10 days, but that was because I was waiting on answers from Forge - but strictly counting "man hours", then that is where two days comes from.
Regarding testing individually, I personally have not, but going by various threads from a variety of other forums, in SCN, vortex, ASN, and somewhere else, then when
either the S3 cooler,
or the Forge cooler have been upgraded on a GTI, the S3 can clearly be proven to provide around 30% efficiency improvement in lower charge temps. The efficiency of the Forge was less clear. Its overall efficiency compared to the S3 was slightly lower, but the Forge remains more constant, compared to either the standard GTI or the S3 cooler (due to where they are mounted). Now, weather that adds up to a cumulative 50% improvement - I don't know, and I doubt it. I would probably guess around 35% improvement, but this will be extremely consistent, even in very hot weather, and when using the air con.
So, the S3 for me was a no-brainer. For the Forge, their reputation had a key part in my decision. I'd recommend either the S3 alone, or the Forge alone, or both together. However, costs will play a key part. The Forge is expensive (you can get them down to £600), but the labour charge should be relatively cheap. On the other hand, the OEM S3 cooler must be a bargain of the century, at around £175 +vat. However, if you are paying for labour for fitting the S3 cooler, then this will be a real killer.
HTH
