OK, it is possible that you soon hit the limits of the drive. But so far all you are doing is making assumptions and then using them as a starting point to move forward. Far better to use the laptop and get some real world measurements based on how you actually use it, rather than starting from ball park guesses. That way you will KNOW how the 5400 drive ACTUALLY performs, whether the track limit is really the HD and not any other bottleneck, and whether a faster HD is actually worth it or not.
When the going gets weird, the Weird turn Pro.