Andy, I had an absolutely stunning winter several years back, on a reasonably hard water by fishing a yellow bird seed with 2.5mls of Hutchies Chocolate Malt per egg.
I'd say that 10mils per pound is very, very high, but I really did take the place apart and I was putting in about 5 to 7kgs a week (another reason that I had such a good winter) and they were eating the lot.
(only about 35 carp in the lake)
Apart from that time, then like you, I've steered clear of most flavourings, prefering a natural food source.
As I've proclaimed on this forum on many occasions, Activ-8 has been catching for me for 5 odd years, so I see no real reason to change.