I have read that cutting a shaft down 1 inch will cost around 1 mph (actually slightly less) of swing speed. That 1 mph of swing speed with every thing else being equal= about 3 yds of carry.

So I would start with how well are you currently hitting the 425 with a perfect shot. going from a 45 inch driver to a 43.5 is going to cost you less then 5 yds with that shot. I would then compare it to your 3 wood to see the diference. (Because the other variable is loft, some people a 3 wood has a more ideal loft for their swing speed then the fancey driver they purchased, if you notice Lowpost has a 13 degree driver when making the comparison but most drivers are a lot less loft)

But the big question is which club will you find the center of the club face more often with. If you are even slightly more consistant with the 43.5 driver then your 45 inch driver then your total driving shall improve and on average should increase. It also should be easier with a 425 club to find a forgiving spot then with the 3 wood.