From Burtons catalog:

http://www.burtonpower.com/parts-by-fitment-type/parts-by-engine-ford/ford-duratec-he/fz631b.html

It looks that is lower than the standard cap, should help bonnet clearance.

What do you think?

Not sure that would make much difference. If you're that tight on space you are probably better to go for the Raceline rocker cover which locates the filler in a different place and makes it a lot lower.