Mine sounds a bit more clattery and takes longer to warm up the colder it is, but I think thats just normal, and figures really. Once warmed up its spot on. I use millers diesel power plus in most tanks and shell diesel extra. I try to give it a good blast on a longish journey regularly too to keep things tip top. I never thrash it till the engine is fully warmed up. Mine has now 36k on the clock and I have had it since 17k.
I have in the past noticed what I initially thought was a drop in power, but I have put it down to just getting used to and familiar with it on a day to day basis.