I agree a lighter wheel is going to help...as this is unsprung weight... but you then have to take into consideration of the tyres weight going up a size..my guess is it would be heavier than the 17" oem tyre so you could end up at stalemate.
but thats not the only factor to be considered when going up a size though...
You have to think where all the weight is on the wheel..which is the rim..by shifting all that weight further out your moving your rotational mass, which inturn requires more power to turn it...
We have had this argument time and time again on the ITR fourm...and yes a larger wheel looks better and probably gives you more grip...but grip and handling are two very different things
Which ever way you look at it increasing the wheel size will lose you slight acceleration sadly too.
If I could lay my hands on some I would love to swap my ST wheel for the 16" Zetec S wheels to see how the car performs..in theory it should be a slightly more compliant maybe at the expense of some turn in feel and slightly nippier acceleration..
Would be interesting to see