I don't think there's anything wrong with the 2.7 at all. It's still a very capable car, and more than adequate for UK road speeds.
Do bear in mind that you won't get as much back on the options as on the car itself come re-sale time. This, along with the more flexible and powerful engine, is why a lot of people (myself included) stretched to the S, which has a lot of 2.7 would-be options as standard fit.
You might be able to get away without the PASM if you are on the 18" wheels. Otherwise, there's the PASM+6 Speed box option to consider.
Personally, if I was going for the 2.7, I'd try to make it as basic as possible.
This means black partial leather interior. Might sex it up a little by adding the leather centre console (front and back) to get leather and stitching by the side of the stereo and by the hand brake. Possibly get the instrument binacle surround in leather as well, again to get the stitching, but you notice this area a little less. Doing it this way (rather than having the whole dash in leather) would cut costs, and the slush-moulded black dash is very good.
I'd probably stick with the leather/alcantara seats too.
And I'd stick with the base sound-system. I hardly ever turn my Bose on!
Lets face it, I want to have the fun of speccing a car all over again! But have fun with speccing yours. Hopefully not a Z4!