I think that there is a bit of confusion here. Power to Weight ratio is purely a 'What power can you put out' Divided by 'How much do you weigh' equation.
So if you take off a kilo of mass (fat, or muscle, bone, fluid, whatever) if you can still put out the same power then you are just dividing by (the old weight minus 1)
If you weigh the same but training improvements mean you can now put out more power, then you will have a better ratio because the power is up and the weight is the same.
That stuff is pretty simple.
More importantly is the question of HOW you lose that weight. If you do it by either over training or over dieting, it can definitly impact your ultimate power output. Both can cause muscle wastage or fatigue. In which case, you would be putting out LESS power and weighing less so potentially ending up with a worse power to weight ratio.