I don't think there is a particular temp at which you start losing power, it's just that the cooler the intake, the more power you make. Hence people running those cryo intakes and peltiers. Whether it's a linear relationship, I don't know.
I'm sure Wilko said something with 3's and 10's in it. So he either said for every 3' drop you gain 10bhp OR every 10' gains 3 bhp. So worst case, a 30' drop would be about 10bhp gain.
Lower temps also allow you to run more timing, which will be a bigger advantage than just the cooler intake charge. Whether the 10bhp is just down to intake temp, or whether this would include winding the timing forward, I don't know.
Also, the stock intercoolers "do the job" but a good FMIC also hase lower pressure drop, so the turbo has to work less to keep the boost up, making it more efficient, putting less heat into the charge. I have seen some custom maps where they've taken advantage of this and upped the boost (keeping the boost at the turbo outlet the same), but I wouldn't go down that route.
For a guideline, if you look for figures for LCRs, have a look at the difference Nibbles made when his LCR was setup correctly - I think his car made 240 odd with the boost turned up, then 260 with it setup correctly.
There's something to be had.
