Speed to a drive is almost identical on whatever speed drive.. the issue is the TC processor is really not capable of full gigabit.
Although drive testing is a bit difficult nowadays.. you need to turn off signing on SMB2 to get best speed.
Which I have done.. but I wanted to give you a quick view of how things are.. so with a bit of optimising I could probably do a bit better on either or both.. but as you can see.. there aint a lot of difference.
To a TC with the original 7200 seagate drive.

To a TC with a replacement WD purple drive. These are slower than green with performance tailored to video.

And in fact the slower drive due to its larger size has better data throughput than the faster seagate.
To do proper testing the disks should be swapped so you can eliminate the effect each TC setup could have.
I don't have time now but I will add a test to the Synology NAS and it will do near 100MB/s in both read and write..
I have done it in the past but cannot see the reference.. here is a close up of the plugs in TC.
Can a ME177LL/A be upgraded to a larger hard drive?