I do that once in a while if I want to slow down for a 2-300m shot, say...I run a Vickers type sling so it's very quickly adjustable and I can easily set it to whatever length is going to work best for the position I'm in. I did it a lot more when I used to shoot a lot of mid-range .308 stuff, around 3-600m.
I don't find forearm mods to have an effect that I have ever noticed. You don't grip a vertical grip like a lollipop unless you have so much gear on the rail that you have no choice anyway...and whatever master grip you're used to will feel fine with the hasty sling approach.