Nas Vs. Building a server with an SAS array

[lsgtechuser]

I understand that the cost of a NAS (with Gb NIC) is much less than building a server with an internal/attached storage array. But I can't help noticing that most NAS's use SATA HDs that run at 7,200 RPMs. How much better would performance be on a file server that MANY people were using if it was an actual server with more computing power, more memory and scsi drives moving at 15k rpm?
Does anyone have experience with this?

I have a multiple NASs and I'm looking to upgrade. If cost weren't much of an issue, would it be better (performance wise) to buy/build a server that would be more powerful and take their place?