IMHO, no, it shouldn't devalue it as long as it is done well and using quality and proven items.
look at it this way.....you buy a Veyron and spend anther £200k on performance mods fitted by a top Veyron tuner, is it really going to devalue it? I think it will make it more desirable.
if, however you spend £200 at halfrauds and cover it in chav gear, then it will probably make it worth less (and the owner would be locked up)
if you went to buy, say, a 22b and it had £20k worth of performance engine and handling mods fitted and tuned by, say, Prodrive for example, with receipts, would you want it?
if the same car claimed £500bhp on the original engine with no receipts and fitted by honest bob's back street shed, covered in cheap, badly fitted rally graphics would you want it?
can of worms