I decided to look at quite a few swings on YouTube. I watched Rory (there's something wired different in this kid, because he's a golfing machine), Tiger, Ernie Els, Fred Couples, lots of Bubba Watson. Then a whole bunch of the long drivers.
Then I watched a whole bunch of videos of the normal pros hitting 105 to 110.
There is a significant difference in what you see, even on videos. The speed difference is like watching a car moving 60 and one traveling 80. It is relatively easy to see. in the case of a golf swing, the amount of shaft flex is very perceptible as well.
I think the issue of hitting 105 versus 127 as far as shafts go is moot assuming you buy the shaft for the 127 mph person. The video with Mark Crossfield seems to indicate why you need a particular shaft for high speed swingers. Yet, it can be used by a 100mph swinger as well.
My guess is that you have the right shaft to use at 100 as well as 125. So, there is no difference.
If you had bought a "stock shaft", maybe you could hit farther and higher? Maybe, when you have a well developed swing of 125 you would need to move up to the $500 shaft.
IMHO, from an engineering perspective, I do not understand why there are $500 shafts, other than marketing. Maybe they will make multiple element shafts in the future with nanotubes for $10,000, but does it make sense to get 25% more distance?
You should check out the LDA, they have many shafts and club heads for fast swingers. I looked at Jamie Sadlowski (5'10" 165 pounds) videos. He mentions that club heads only last a couple hundred drives or so. Impressive. He swings 150mph, and has the genetic disposition for speed.
I'm not sure if everyone thinks I have a stock shaft in those videos or what, but that is a club that was made specifically for me years ago. Its a KZG 360cc with a 47"UST PROFORCE X stiff shaft, I haven't played much over the years, and just started getting into it more. Now the head has a smashed surface to it, and my instructor said I needed a new one, and figured while I was at it I would look for a new shaft, and shorter one at that( looking for accuracy). From my personal experience I've never been able to use a stock shaft found in box stores(R-S-X), they are always way to "whippy" feeling, and that's coming from my perspective, friends I was shopping with, salesmen at the stores, and club fitters. And when I have owned/used them, I will almost always snap the head off within a short amount of time. I've always been told I need x to xx stiff shafts. I started this thread to just ask for advice, on whats a good shaft that's not too expensive. I do know there is a huge feeling difference when I swing a club with the right shaft in it. Thanks for the help