Geometry question: shorter stack with same reach? I'm planning on building a new bike soon but I'm quite conflicted about choosing a frame. For now I'm riding a GT Grade AL 2015 that I use everyday to commute but since I'm planning on doing some bikepacking tours overseas, I want to have a lighter bike. Buying a carbon GT Grade might be nice but they don't sell them as frameset and the specs they are offering are... Uuuh... Undesirable.

Now then, I might buy a CX or gravel frame, both are okay as long they can fit at least 38C. The CX frame that I have my eyes on has a stack/reach of 561/374 mm and BB drop 57 mm while my Grade has stack/reach of 578/373 mm and BB drop 70 mm.

Would the differences in stack and BB drop make a noticeable difference? I would say I wouldn't mind a bit of racier position since I also have a road bike but I just need some more inputs.
Also the CX frame costs like 50% less than the current gravel carbon frames that's why I'm leaning toward it. Plus higher ground clearance might mean I could fit 650B wheels there (hopefully).
Don't just look at the stack and BB drop, consider the steering geometry as a whole (chainstay length, BB drop, head angle, front-center [BB axis to front wheel axis]) which will affect how the bike handles; and the cockpit geometry (stack and reach) which will affect your position. Cockpit geometry and steering geometry are interrelated obviously, a bike with a long front-center will have a long reach for instance.

The Grade geometry is really long chainstays, long front-center and a slack head angle (FYI, I own one). The stack is quite high too. If the CX frame you are looking for has a very high 57mm BB drop I bet it has traditional CX race steering geometry as well: short wheelbase and steepish head angle. You'll very definitely feel a big difference between the Grade and and a trad CX frame like that.  

I'd really hesitate to use a race geometry CX bike for touring. The high BB moves your center of mass higher up and the short frame and steep head angle make for quick steering - not what you want for loaded touring. I don't think you'll want a low stack that results in a leant-forward rinding position for long days in the saddle either.

There are plenty of gravel frames out there that are shorter, have 1/2 degree steeper head angle and a little less stack than the Grade. That's what I'd look for for a touring adventure/gravel bike.

BTW you don't need a high BB for 650b wheels, as the point of using those is to run larger tires (45mm+) than you can with 700c rims. The difference in whole wheel diameter between a 700c with 25mm tires and 650b with 45mm tires ends up being quite small.