My MS uses an external USB DAC (Rega) Alex, all current info here
http://www.bluedive.co.uk/musicserver/index.html
I have tried virtually every conceivable combination of hard drive/NAS installation possible and have not found anything to better an internal SSD, apart from a flash drive, which gives comparable results, but you can't store 700GB on one of those! - but we are getting pinickety here and its a dedicated server, not a multi-purpose computer. When I tested those years back I quickly found out they were not up to the job for my needs and it has been done to death quite a bit to be honest.
I am not surprised you have not noticed that much difference in a laptop, I found the further up the scale I went to more noticeable some changes became. There is too much other stuff getting in the way in a laptop (electrical noise) to really get the benefit from changing smaller items. But if it makes no difference in your system then stick with that if you are happy? I was never content using a laptop or any type of general purpose computer for music playback, it has to do one thing and one thing only.
I'm a tester, I don't really take anything anyone says as gospel until I try it myself and see what the results are. If I like it I keep, if I don't I bin it and move on and sell it - eBay has taken a lot of my testing trial gear
It can take me a long time, but I'm never happy until I try it myself and I tend to use information from others as a guide and for inspiration only