Ahhh interesting subject and i had / have exactly the same issue on my 3dr cossie.
so i fitted the MK beam (i bought it secondhand) with new black series powerflex beam bushes back in the summer and bam the box section part of the beam was hard up on the floor. I compared my new bushes with an older set of powerflex bushes and the dimensions have changed. The overall size is the same but with less meat at the top of the bush so the beam sits higher up on the bush and thus against the floor just like your solid bushes.. I have fitted my old style black series polybushes that i had in my standard beam and now it clears but is still very close. I presume the design changed slightly to sit the standard beam higher up keeping things more aligned on a lowered car, but that's a guess and really i have no idea.. I am also concerned that it still might touch and cause vibration through the floor requiring a further bit of spacing. Will find out when i get to drive it but currently waiting for a set of trailing arms to be modded. My advice to anyone else thinking to use this MK beam on a 3dr cossie is dont.