I decided to install this Seymour Duncan Invader SH-8 into a really cheap 7-string I picked up off this very website, the logic being that I could dramatically alter the sound and turn my new sub £100 guitar into a screaming rock machine.
Did it work? Kind of, is the only answer I can give. As suspected the stock pickups in the guitar were distinctly mediocre, lacking in clarity with some noise and hum.
I installed this pickup in the bridge position an immediately noticed a substantial difference. There was much great clarity and dynamic range coming from the bridge after installation and the sound was crisper, brighter with a higher output.
However it wasn't as tight as I might have expected. There was still some muddiness at the low end. It's quite a bass-y, scooped pickup anyway - best suited for metal as the name suggests! The clean sound was actually quite warm for a bridge, rather than glass-like sterility of some bridge pickups. Overall I'd say its quite organic sounding.
Obviously a pickup is only one part of a guitar's sound and its worth remembering that natural sustain in the wood, plus the quality of the rest of the electronics within has a substantial effect on this (which I believed might have been the case with this Invader, as I've heard it absolutely scream in other guitars). All in all it can really improve things but can't work wonders. If it was on a slightly more expensive guitar (probably around £300 mark) I reckon you'd get some killer riffs out of it.