The Hoppes rest has several shortcomings. It is too light. and when you combine that with an inadequate spread between leveling points, and relatively high bag friction, you get a rest that tends to move as the gun recoils. Obviously one cannot be sure that the movement is the same for each shot, thus introducing a variable that will enlarge groups. Another weakness in the design is that the bag plate is secured to the top of the threaded elevation screw with a set bolt that has a small plastic head that is designed to be tightened without a wrench. Tightening it by hand, it is almost impossible to get it tight enough, so that the top cannot be made to rock on the elevation adjustment shaft. If one is on a budget, all of these issues are fixable. You can mount the rest on a heavier, broader sub-base, and either replace the set bolt with one with a conventional head, and tighten it with a wrench, or use a pair of pliers on the existing knob (carefully). The suitability of your bags depends to a certain degree on your shooting style, and this is partially determined by the design of your stock, and your rifle's balance. The crucial test being whether you can keep the cross hairs perfectly still as you are pulling the trigger. Better bags are usually more secure in this respect. Putting a number on all of this, is difficult to impossible. You may have other issues that prevent your rifle from shooting at a higher level, making it difficult to weight the value of one thing in the system. I am reminded of shooters who, after switching to a Wilson neck die and seater, find that their groups have not improved, because there were other issues besides reloading dies that were holding them back. This is not to say that better dies will not show an improvement, just that the whole system has to be performing at a high enough level for it to matter.
Years ago, I can remember when I horse traded into my first rest and bags (Hoppes) and spotting scope. They were a wonderful improvement over what I had been using. At the time, I was shooting a 788 in .308 with the cheapest 4X scope. Having an adjustable rest, with bags, and being able to see bullet holes from the bench was huge. I was having as much, or possibly more fun then, hammering my loads together with a Lee Loader as I do today, with a full, load at the range, shoot over flags, Benchrest rig. It was all new, and I was learning by doing, and comparing notes with my friends. Generally, after I worked things out, I was able to keep my groups (5 shots) under 3/4", and I was proud of that, given my sighting equipment. It wasn't till sometime later when I came across a Benchrest shooter who was practicing at the same range, that I became aware of more sophisticated equipment, and what could be done with it.
Kinda' hard to quantify on the terms presented!
