Craning of neck
 

I recently flipped my 7-degree stem (from upward tilt) and dropped the handlebars down by a single spacer. [If it matters the headtube angle on frame is 71.5 degrees]

It seems as if my average speed relative to wattage has increased. But my neck and upper back are sore and stiff.

What should I try first?

a) flipping the stem back to +7 degrees

or

b) raising my bars by one spacer?

I just want your vote as to what to do first.

Leave it where it is for a few more rides.

This.

Can't say without knowing where you were and now are in terms of relative seat and H/B height.

However, given how far you dropped it, moving back up by the spacer (what thickness spacer?) will make much difference.

So, might as well wait and see if your neck gets used to it.

Seems like you've already made up your mind between two options that are essentially the same result.

So I'll just add that contemporary wisdom is to never do anything that seems challenging, difficult or risky. If it's not immediately easy, it's just not worth pushing yourself to pursue.

Now, you may never get better at anything, but at least you'll be comfortable and safe.

You forgot an option:

c) do some yoga every morning

Flip it back up and bend your elbows more when you want to be a bit more aero is also an option.

Alternatively, focus on rotating your pelvis forward so your back is straighter with current setup.

There are many options, you should do what fits you best.

Why did you flip and lower your stem?

I went from 30mm of spacers and 7° upward stem angle to 0mm spacers and -7° stem angle, and I did it for the best reason possible: BECAUSE IT LOOKS COOL! And because I WANTED TO!

I'm naturally flexible and I work to keep myself limber, but I was pretty impressed at the stress it put on my neck and back. Took 3-5 rides and some rest days to sort it out, but I'm sure I'm stronger now. And I look much more ... something.