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It must make it more aero - you've got a frame that's, say, 700mm bottom to top versus one that's say 750mm bottom to top. With the taller one you then still have a stem on it but it's -17 or whatever to get rider back into position. The taller frame is always going to have a greater frontal area. I've never used a handlebar bag. I try and get most of my stuff on the frame or out the back rather than on the bars.
hippy-
i am not so sure. if the stem on the smaller frame is not such that it makes the stem parallel with the ground then you have rise, and if you are in the same position, roughly the same frontal area.
you can also reduce stack i'd say by about 5-10mm via headset changes. i've had a shitty slamthatstem.com cover on mine for a while (i think the stock headset cover adds about 10mm) and have ordered cane creek's slamset which ought to work similarly.
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If your stack (at the bars) and BB drop is the same between the two frames then theoretically the frontal height is exactly the same. As long as the sum of the stem angle + headtube angle isn't greater than 90°.
Not that that makes it a good idea necessarily
zmjones_
frankenbike
that is exactly what i do with that frame.
@hippy i don't think it really makes it much more aero. with that hta a -17 stem looks almost flat. fwiw i haven't had any problems dangling all manner of fred shit off of mine. an added benefit (actually) is that you can have more tire clearance for hanging a big handlebar bag off the front.