Sam__ said:whats the performance difference between the 90 and the 120
Roughly the same at the same level of airflow iirc but the 90mm fan will be louder than the 120.
I'd imagine that using a fan spacer could also reduce the impact of the angle of a fan adapter. You could also duct the fan to the fan adapter (the adapter being mounted to the heatsink).
Or you could save yourself the trouble and buy an Ultra 120. By the time you buy the adapter and ducting, you'll have probably spent more money than just buying the Ultra 120.