just another thought... ***For discussion only***
wouldnt the factory plenum be designed to distribute the air flow from its orginal position in the best manner possible ?
by changing the inlet, that would have some affect on the air flow characteristics
Dont jump at me, i am hear to learn too and provide discussion.
and TonyGtst i think aftermarket front facing plenums such as the Trust item would be flow tested and made to suit the forward facing application... hence lots of R&D