Is it normal for me to be getting so much lag on a Maxed out GTAV? I mean, everything MAXED.

No you shouldn't be lagging really...do you have any video of it or anything? What res are you playing at and how much fps drop and when it drops it lags? My fps fluctuates a lot in that game but no lag.

I play gtav with everything maxed and even at 4K with 40-45 fps it's really smooth even when it drops down to 30 fps with grass at ultra. But like modern said you can tweak a couple settings with not much difference to visuals, for a little extra performance.