Upload & Sell: On
Both ways should work out fine. You can still keep the original 5TB and a new 5TB connected to the computer. LR will go get the files from wherever they are. It won't slow LR down. Only thing that would slow things down is if you don't have both connected and have to plug in the old one when you go back to work on those files.
If you buy a new set of 8TB then you could copy over and use the 3TBs, then start using the old 5TB set again (after wiping them).
But whatever you choose there won't be any performance hit on LR. I currently have 3 drives that LR is referencing (2008-2014, 2015-last year, Current year). They are all housed in one DAS TB enclosure but you don't have to have them like that, just have them plugged in via USB.
I would just look at price per TB of buying a set of 5TB vs 8TB. If you buy the 8s then you have 13TB to work with...should last a long time. If you buy new 5s you have 10TB to work with which will also last a long time.
A quick look on Amazon shows Seagate External BackupPlus 8TB at $25 per TB and 5TB for $24 per TB so very equal.