I think it is dependent on what you call "large" orchestra.
Did you connect the new 7200rpm harddics internally or do you still use an external USB2.0 case ?
I think USB 2.0 will be more a bottleneck than the 7200rpm, although as mentioned before, access time is also an important factor.
But the most important point is how many instruments you gonna play at once, maybe there also is something additional going on with other softsynths, samplers etc.
Also : What audiointerface do you use ? There can be large differences in CPU and overhead demand between the models. If you get clicks and dropouts, try using a larger latency.
IMO you should be able to basically make music with an i5 laptop and a fast conventional HD using the Special Edition.
But hey, there are people here on the forum discussing about 3x Raid0 WD Velociraptors or multiple SSDs. It really depends on what you want to do.