Re: double haul question
I can't see how it's possible to throw ANY fly line over 100' with anything other than an excellent forward and backcast.
Are you basing this upon the fact that you can *see the backing on your reel and you know you have a 100' fly line, or have you actually used a long meter tape to verify your distance? I've been out with people who've said they "can cast 100 feet" and then found that they're nowhere near this mark -- with snake-like loops of line laid out in front (and often a collapse at the fly end) and my tape spitting back closer to 60-70 feet more often. I haven't watched you cast so I obviously can't determine this, but I just can't imagine someone with an admittedly bad forward cast hitting 100'....or really even beyond 60-70'. That's just not been my experience. Beyond 70', to me, requires a pretty solid all-around cast.....longer distances require more attention to minor details in timing, loop size, hauling speed, trajectory, etc.
Also, with a mod/fast or fast 5-weight you should be pretty close to your max distance with ANY rod IMO. I don't find that there's much difference in my distance from my 5-8 weight rods, everything else being equal.
Do you have a video of your cast?
My wife has some issues with forward creep (and resulting tailing loops) as described by silver above -- very good advice from him as always. He's helped me quite a bit in developing my own cast.