Ben.
My understanding is the K27 turbos are more reliable and designed for our air cooled cars but they take a significant supply of oil to keep the bearings cool.
The GT's and others offer more design flexibility with many more wheel choices and they are were originally designed for watter cooling. I suspect the ball bearing style cartridge may reduce the cooling requirement so some degree. The plus for a retrofit is they do not require the oil flow the KKK dose.
Thus, though there are lots of reports with the non KKK's living a long life on a 911 motor, the KKK might be a more durable choice over the long run.
For a 911 motor making about 350 HP the K27-7200 is a near perfect fit. Especially for CIS cars as it matches the fuel deliver potental very well. It has the right size hot side and the compressor is well matched so it comes in fast & hard and dose not get soft on boost until the higher 300's HP.
The HF and K29's typically keep the 7200's hot side (7006 hot side for track cars) and put a bigger compressor wheel with it. They can start boost earlier as the compressor moves more air faster. However, because of the change if the leverage or ratio between the hot an cold side, it takes a longer duration to reach full boost. This dose not mean it will reach full boost later than a K27-7200, it just takes longer from boost onset to say 1 bar. With the bigger compressor wheel it can support over Hp in the high 300's and even well over 400hp with out boost falling off.
On my old C2 3.3 Turbo the HF turbo seemed more civil and came up on boost in a way it felt more like a big normally asperated motor. I ended having issues with it and went back to a K27-7200 and it came in much harder and faster which more fit my style.
The issue with the HF and K29's that use the K27-7200 hot side is at the higher HP level they acces, there becomes a need to vent more exaust out the Wast-Gate circut. With most of the 930 type exausts the WG circut is close to being at its limit and this can result in boost creap. I had to to go to a .7 bar WG spring because of my B&B style headers and the biger turbo and I still saw 1 bar in the higher rpms.
Your HE's with twin WG's should be a soulition to the overboost problem. However, for a track car it is better to just go to a larger hot side turbo and live with the higher boost onset.
