After a bit more research it looks like my adapters are old and don't support some of the newer tech.
Seems I can get a bit more speed by getting a dual band adapter that supports 802.11 n and ac, although from what I've read 'ac' can actually be slower than 'n' due to interference issues.
Quite surprised there's differences in speed between different 'n' class adapters, thought they all operated to the same specification so the speeds would be identical.