Keep the "medium" ones as-is, shave down the tallest ones, and shim up the lowest ones.
Casey

Maybe shim them all to the level of the most forward one? Not easy to plane down a stud.
Or perhaps you could 'sister' on 2x2 pieces to bring them all out the same?

Sistering the existing studs out to a level plane may work, but 2x2's or 2x3's available today are likely to be even harder to use than 2x4's because of twists, etc. Many builders today routinely use steel studs in places such as the kitchen where a level wall is crucial for fitting cabinets, etc. If the wall is relatively short, sistering may be the best option. For a long wall, fitting steel studs one by one and removing the wooden ones as you go would be best. The presence of wiring or plumbing in the wall would make this option quite a bit more complicated, but not impossible.

It sounds like my husband and I did it the hard way....it worked well but it was a little labor intensive. I did it during one of my summers off. After we located which stud or studs were furthest out we shimmed the rest. I would put a new 4X4 even with the strings we strung and scribe the new 4X4 to match, cut it with a jig saw and glue into place. Rip a new even edge on a jig I had set up and move to the next one- flip to other side when possible. We had so much stuff in the walls this was much easier than sistering and cheaper too- my labor was free!

Heather

This backsplash is sandwiched between upper and lower cabinets that are only about 13" apart from one another. We already have new countertops installed too, so I would rather not use a saw or jig anywhere around that area! Someone else suggested shimming with strips of thin wood paneling so I'd have a flat surface when finished. I definitley can't replace the studs. In my head, sistering the studs seems easier than a bunch of shimming. I am going to try shimming first since it will be less costly...I'll let you all know how it goes! Thank you all for your advice!

-Amber

I rip sections from wider 2x limber (often 2x6 or 2x8) and then sister them to the sides of the old studs to create a new flat plane.

I use a couple screws to attach the new wood and set the plane, then go back and add additional nails with a nail gun.

If you use a laser that projects a vertical line you can make a spacer to set all the new studs in a common plane.

You can use temporary "studettes" as a guide for a router (like a laminate trimmer with a tiny baseplate footprint) to plane down the high ones. I like my plan because it maintains the existing plane of the wall without shimming all the rest out to match the highest stud, which changes the position of the whole backsplash. And you only treat the highest and lowest, any that are true need not be dealt with, so there could be some efficiencies there.
Maybe the simpler way is to float the wall with mortar before tiling. Durabond 90 would accomplish that task.
I think engineered wood studs would be a heck of a lot better than introducing steel studs (you must be talking the heavy-gauge ones) into the kitchen framing. Light-gauge steel studs would be pretty near worthless for cabinet hanging for a typical cabinet installing crew. They would be flummoxed unless special preparations had been laid, like plywood sheathing or blocking to screw in to.
Casey