I think the problem here is how you nerf the character. It shouldn't be about straight up making them worse. It should be about changing how you play them, or in this case how you deal with him. Like for example (this wouldn't work, its too drastic), you could make it so PLs illusion from doppelwalk is a yellow one, not a clone. This is a simple thing that makes worse players understand and deal with it easier. Like "I know he's invis now, I should dust!" But it wouldn't make too much difference to pro players, you can pretty much tell by judgement when a PL uses his invis, so would dust anyway.
That ideas not great, but the idea is you can change how the hero works a bit rather than changing how good he is stats-wise etc. I'm sure Valve/icefraud could do it. There are many other really good strong heroes that are balanced across a lot of the tiers. Also I don't included really low pub play, but I would say PL is not great in mid-high either.
