you really have no clue!
Leinster have an average attendance of 14727 which is greater than all in the SPL but the OF and Hearts. As was said, Edinburgh was awash with blue and black during the weekend of the H-cup final, and that was replica tops. I'd be interested to know how many tops Motherwell shift in a season...
incidentally, Cardiff, have an average of about 8600 (so 6th in SPL). Also, how many SPL teams could go to their national stadium for the odd game and sell it out? Wasps' highest home league attendance is 81000 (set last year i believe).