I've really hacked up my Nimbus Ozone. Obviously weight savings were part of the reason, but I found the stock frame way to stiff. The range of sizing is unmatched by any pack in the world, I believe, so I didn't want to lose that.
I've got the packbag itself down to about 1lb. 6oz. From there I add a stripped down set of shoulder straps to a hacked mini frame to get the whole thing coming in just a hair over 2lbs. Considering the stock one is really about 3.5lbs., this was quite a hacking.
I recently got a Meridian for cheap, so now I'm willing to hack a little further on the Ozone. Shorten the collar, remove some of the belt clips and straps since I use this pack without a belt. Probably will get it a little under 2lbs.
I have 3 frames for it - the stock full frame (12 oz.), a shaped frame that retains all the functionality, just less stiff (8 oz.) and the mini frame I use without a belt (4.5 oz).
I'll take some pics of the frames and post them up when I get a chance.
I've tried dozens of packs over the years and always come back to this series for the fit and comfort. Especially for big guys, I don't think there's a better pack on the market despite its 6+ year old design.