I'm looking at upgrading my current road bike (a Scott CR1), and the new bike I'm looking at has about 1.3cm more drop from saddle to bar. The reach will be comparable (within 5mm or less). The seat tube angle on the new bike is 73.5 whereas the old was 73.3, so if I draw a line directly from the top of saddle to the centerline of the handlebars the new bike has a distance which is about 5mm shorter than the old one. All this to say, the only thing that seems significant to me is the 1.3cm diff in stack. I'm trying to order direct so I can't try before I pay. Any thoughts on if this extra 1.3cm is going to feel earthshakingly different? I'm assuming it goes to my flexibility and how much I can flatten my back? Thanks!

I don't think you'll notice it. I go back and forth between two bikes with about 8 cm difference in drop. The difference isn't noticeable.

By the nature of your question, it assumes that you cannot zero out the differences using whatever saddle rail or headset spacers are available to make the fitting specs of both bikes the same.

Depending on how finely tuned your current fit & position is, 13mm more drop can make for a meaningful change. Transfer a 10mm spacer from below the stem to above the stem of your current bike and try it out. If you’re already riding with the stem slammed then you probably know what you’re doing and can figure out your new balance point with a few minor adjustments.

Just depends on your body. If my bar drop is anymore than 7cm -- and I mean by just 5-10mm -- my power output suddenly drops 20% - 30%. But, everyone is different. (If it's only a 13mm difference, all you'd need to do if you want to keep it the same is add a couple spacers and/or flip the stem.)

already limber? bend over, touch the floor with the palm of your hands?

The floor? I can't even touch my knees.