Normally if you ask nicely BK will do their P90s in HB size and vice versa.
As you correctly assumed, your choice comes down to the Pig90 and the Stockholm. The Supermassive might work on the count that it is brighter, but I think it looses with the rest of things.
Question now is how extreme you wanne go. My understanding is that the p90 is somewhat more extreme than the SH on all counts. I think your biggest concern should be the amount of low end you get, as all these PUs will deliver quite a bit of that (fat p90s really are fat, but defined, love it). If you would want to go full extreme you could ask BK if they would make you a Ceramic Pig90 (dunno how the chances on those are these days, but they are a lot better than on most other custom stuff), which should definetly be tight enough, though to be fair, the others would be too.
Generally I think you should shoot them a question and if they donīt mention a C-Pig90 themselfs you can maybe nicely throw the thought in there and see if they feel it might work and are willing to do it.
