The number of cells determines how well the cat flows the exhaust gases. A 100cell flows slightly better than a 200cell at the expense of possibly failing the mot although where I bought mine I was told that they have had customers that passed the mot with 100 cell cats no problem.
£85 for a sports cat is dirt cheap, so would you definatley see gains, i have a catback at the mo hoping to order the 5zigen mani in next week or so, would you say go ahead and oreder the sports cat