50 Feet of RG-213 is going to have about 2.5dB of loss at 445MHz.
50 Feet of LMR-400 is going to have about 1.3dB of loss at 445MHz.
Losses will be less at VHF, but you get the idea.
So, looking at about 1dB of reduced feed line loss if you keep the length the same. Probably unlikely you'd hear that 1dB.
LMR-400 is a "better" cable if you want to look at it that way. 1dB might be a good thing, but it depends on what your expectations are. Installing new coax with properly installed connectors and weather proofing everything that's outside won't be a pointless exercise. You can do the calculations yourself here:
Coax Loss Calculator
I'd agree with the others, keeping the antenna up high is going to help more. Also, LMR-400 isn't the Holy Grail of coax, it's good stuff, but don't limit yourself to just looking at it as the only option. 50 feet isn't a lot of cable, so a little bit of extra cost won't add up to much.
On the flip side, chasing 1dB of loss in your cable is often something you won't be able to hear. In other words, spending money to improve system losses by 1dB can be kind of pointless, unless you are really on the fringes of coverage.