I might be able to be of some assistance.
Back in Jan I bought a Razr Fit driver, 11.5* A flex. I'm only 5' 4" and always struggle with driver length. So I had the Fit shortened to 44.5". Swingweight was still around a D4. So I had it shortened again to 44" and with an undersized Winn grip was still at a D0/D1. Driver felt great but my swing speed improved and the wet noodle stock shaft had to go.
So back in May I took my driver to a Callaway fitter for a shaft fitting. He pulls out all the standard upgrade offerings and I told him, don't you have anything shorter and he said no, Not really. At the same time we both said....let's try a 3 wood shaft in it.
He only had 2 offerings in R flex, the stock Callaway shaft and the Diamana Blue 65 gram.
The stock shaft was ok but the club was doing some very weird things on his simulator at impact. We put the Diamana in and it was love at first hit. Playing length of the club is 43 3/8" which is perfect for me. I haven't had the club swing-weighted but my guess is it probably falls just short of a D0. I'm a slower swinger (about 88 mph on avg) and the shaft flex feels perfect for me. I like a little tip action and this has it. I know most will say that a 3w shaft in a driver should play much firmer than the stated flex but I do not find this to be true at least with my setup. I have a feeling though that this is because the Fit uses a shaft adapter which adds some weight (it might be marginal) but I think this helps with retaining the proper feel and even though its tipped adds some weight towards the tip end to help out to add some flex.
How I got the idea was a couple weeks before my fitting, I had gone to a demo day and TM was there. I asked the TM rep if I they had any driver shafts shorter than normal and he said no. I asked if I could chuck a 3 wood shaft in it and he said sure and put the stock RBZ 3w shaft in the R11s 12* head and that combo felt really nice too.
The key to any shorter driver and getting it to feel right is normally a heavier shaft and heavier head. If you can swap out weights in the head it makes it all the more easier to build such a club.
You are correct though, the added weight on the head will make the shaft play looser. if you fail to add any weight and just shorten from the butt end the shaft will then play much firmer (I learned the hard way...my last driver, Burner SF 2.0 was a Sr flex and I shortened it down to 45" without adding any tip or head weight. We then put it on a scale and it then played as a Stiff flex.
If you use a normal driver shaft you might want to think about cutting the shaft down in equal parts from the tip and butt sections of the shaft. I know my buddy added weight to his r9 Supertri and cut from equal sides until he got it down to 44.5", He plays a stiff flex and says it still feels the same prior to shortening the club.
