I was wondering if I should buy a sealed sword with ASI med on it for 2.5-3k ce to re-make my GF for LD. Would it make a difference?
A Good Choice?
Would it make a difference, compared to what? You're wondering whether a Gran Faust with ASI+2 is much better than a Gran Faust with ASI+0? It's going to be about 7% faster (data here). That can help a bit in Lockdown, yes. It can help a good player defeat an equally good player. I don't see it making a bad Lockdown player instantly great, though.
Let's say you spend 2,500 CE, with CE at 6,500 crowns per 100 (I haven't checked prices recently). That makes 162,500 crowns. Then you need to upgrade to 4 stars, heat at 4 stars, upgrade to 5 stars, and heat at 5 stars. It's going to take a while. Its total cost depends on things like how many radiant fire crystals you have lying around.
Alternatively, you could try Punch. Assuming that all eight UVs are equally likely and +1 occurs 2/3 of the time (similar but not identical data here), you expect to try about 8 * 1 / (1 / 3) = 24 times, for a total cost of 480,000 crowns, but with the possibility of getting ASI+3 or ASI+4, not just ASI+2.
So that's how the economics work out. Whether this is a good use of your resources, or maybe there is some better use, is something that you have to decide. Good luck.
The more ASI you have, the more opportunities of attack you gain. It also helps with evasiveness as it increases the speed of your attack animation. The least amount of ASI you should have is High to not end up as a slashing bag to the faster striker, so unless you're hitting High with your gear already, buy the Sealed Sword and create a new GF (or Faust).
As a bonus, you get the ASI in PvE too, so isn't that nice.