I would say this totally depends on the storage drives used in the server. iperf3 doesn't interact with those at all so it might have a slight advantage .
also, 10Gbit (or 8.25Gbit/s) vs ~1GB/s is almost the same speed already? line speed is usually given in bit/s, disk transfer speed in byte/s, there's a factor of 8 between the two https://en.wikipedia.org/wiki/Data-rate_units
Thankyou gents, my wording is a bit questionable but you get the idea.
I have looked further into it and have adjusted some mount and export options and performance has improved to 7.9GB to and 5.1GB from nfs server so pretty happy with that for now.