No.
Can't really put a percentage on it. Depending on the nation set up, it could effect them minimally or a hell of a lot (ie if they're improvement swapping and lose the population).
Currently there are 30,795 nations.
to get to 5GRL, you only need to nuke 514 nations in the period of 30 days. That's 17.13 nations a day - easy. To put this in Perspective, Great War I saw between 300 and 350 nations nuked. That was over two years ago. At that time, the actual GRL was like 11 but there was a cap at about 3.
GWII and III were largely non nuclear affairs (GWII was certainly not the largest nuclear exchange ever).
I believe the first fan war saw the launch of about 1500 nukes (guestimate and poor memory) spaced out over a period of time. I don't believe GRL had been reimplemented for that war.
The Unjust war - it saw significant use of nukes. I'm pretty sure it did max out and stayed rather high for a couple of weeks. Remember, if you hit the maximum number, it's going to stay until the total over the entire 30 days is below the required nukes. Even if there is a relatively small but elevated, continued barrage it could be enough to keep it maxed.
I'm unsure how the SDI will effect it, but it probably will make it more difficult for the GRL to rise. I still do not doubt that a major nuclear war went off, that it would cap out within a day or two and stay there for a while.