Adjust on export

Asking again- Why won't "sync" work?

If you want to "increase my temp by 500 and decrease my exposure by 10 for everything..."

My method would be-

1) Create a Collection of 300+ photos for Lab-A

2) Select all 300+ images in Library view  [Ctrl+A]

3) Press [Ctrl+'] to create Virtual Copies for all 300+ images

4) Filter for Virtual copies intended for Lab-B  (Attribute-  Filter bar)

5) Adjust Temperature and Exposure with the 'arrow' buttons in the Quick Develop Panel.

Thus- Originals = Lab-A.   VCs = Lab-B

Then you would Export (as sRGB) Originals and VCs separately.

Are you using the exact color space the printer specifies?

I run into this same issue printing to Metal Prints. The Metal Substrate is about 1/3-1/2 stop darker than the paper and canvas to which I normally print. It has a slightly warm tint also. 

I found that creating a tone-curve preset accomplishes both the brightening and the color correction on top of what I've already edited. (Disclaimer: I don't normally use a Tone Curve, so it is always unadjusted.) If I am printing to metal, I apply that preset, and life is good. 

I suppose you could create an off-the-edge reverse linear gradient that overlays the entire image and apply the edit there to tweak if you've already used tone curve.

Nothing in the Export dialog gives you this option. Edit adjustments need to be made in the Develop module or the Quick Develop panel of the Library Module. You could make use of Virtual copies and adjust them with Quick Develop adjustments (that 'add to' current edits in an image.)

Not sure what you mean by "so sync will not work".  Do you mean that each image would require independent and different adjustments?