Gripping the magwell is bad practice, so I hear. Doesn't give you much control over the front end of the gun, the placement of your off hand is too far back.
It's not for everyone, but I tried a magpul angled fore grip (the AFG2, to be specific). Didn't like it at first install, but after acclimating to it I REALLY like it. Less than 3 ounces, could be something to consider. Also, just some ladder style rail guards that weigh next to nothing can improve your grip and comfort over the front end. If you are set on having a vertical foregrip, magpuls RVG is pretty light.
Just out of curiosity, what quad rail do you have? more specifically, what does it weigh? Just a quick check on Amazon and all the cheap quad rails (I'm talking as low as 17.99) are 9-12 oz, those that give an actual product wieght as opposed to shipping weight. And those are aluminum. The expensive ones are not any lighter. Not sure changing your quad rail is going to save you much weight and might not be worth doing unless you still just want to upgrade to a higher quality (supposedly) quad rail anyway.
Now if you go to a polymer handguard, like the magpul MOE one (6.8 oz) and didn't add anything to it, THEN you could be saving up to half a pound maybe just on the rail. add a rail section to the MOE and an AFG2 and you've still saved a quarter poundish, plus the weight of the grip you have now.
Just spitballing options for ya.