FYI the only physical difference between the stock 150 and the 180 is the size of the intercooler, the Mountune upgrade for the 180 to take it to 200hp is a new airbox and software, they also do the upgrade for the 150 to take it to the same power which includes the intercooler in addition to the above, cost is around a grand plus fitting though.
From memory, the cheapest option for the 150hp version is a Celtic tuning remap (200hp) at about 250 quid, followed by a Bluefin (180hp) at about 400.
I went for the Bluefin option (180hp) and performance is virtually identical to that of the e46 330i (touring automatic) it replaced, economy is around 40mpg vs 23mpg for the BMW so no complaint there!