leather treatments on hybrid leather/GoreTex boots?

In a warm but well ventilated place, I apply two coats of Limmer boot grease (they still call it grease but it's no longer greasy) to the leather surfaces with an old toothbrush. After it's absorbed into the leather, the whole boot gets two apps of Tectron boot spray with plenty of drying time after each app if possible.

The Limmer compound is designed for leather, and will allow it to 'breathe.'
The Tectron is a DWR (Durable Water Repellent) treatment that will allow fabric to breathe after application.

That's the approach that has worked best after trying just about everything except Granger's, which is no longer easy to find in the US. I chucked SnowSeal long ago, because it was next to worthless.

The odor from the Tectron used to be pretty off-putting. They changed the formula to reduce that, but now it's not quite as effective as before. If you don't have a heated area that's separate from your living area, like a mud room or basement, that's a problem. I use a mud room that stays well above freezing for drying.

Keep in mind that if any of the oil/grease/wax makes it through the outer layers to the Gore-tex membrane it might cause it to leak. Oils are notorious that way.

An externally applied leather treatment should have no impact at all on the GoreTex liner in a boot. That stuff isn't going to make it through the leather. It's just like a DWR treatment on the exterior of a jacket.

Yes, the leather needs lots of SnowSeal if it is going to handle wet conditions.
Of course, expecting the GTX to breathe through well-waxed leather is a bit of wishful thinking.

Don't expect all material combinations to make a lot of technical sense, but they look good in the marketing spin.

Cheers

I've owned several pairs of fabric/leather boots lined with Gore-tex. With all of them, I did nothing whatsoever to the leather. On all of them, either the fabric wore out, or the sole wore out, or stitching failed before I had any issues with the leather. So the only reason I can see to put anything on the leather of these boots is to reduce how much water they absorb, thus speeding drying and reducing the rock-hard frozen boot syndrome on colder trips.
As to breathability. I've always assumed that none of those boots had any to begin with. Too many layers of stuff for any meaningful breathability.
I also had one pair of gore-tex lined full leather boots. With those I did treat the leather, and in that case it was the sole that wore out first. Though by that time the waterproofness of the Gore-tex liner was long gone.

I only use goretex boots in snow and cold, so the reduction in breathability doesn't matter too much to me. Yes, the insides still get damp on multiday trips but it's a LOT less than if water seeps through.

"So the only reason I can see to put anything on the leather of these boots is to reduce how much water they absorb"

Don't forget, treating the leather is not just about making it water proof. All leather requires cleaning and maintenance. Otherwise dirt, salt, sun exposure and other "gunk" can cause the leather to dry out, not be be as supple and ultimately not last as long.

I treat my leather hiking boots (used for trail work, mud season day hikes, around town in winter) and my leather ski touring boots once a year. Usually in April. I've had good luck with Sno Seal personally but I realized everyone's experience is different for sure.

Here's a nice little FAQ from Danner GTX boots that probably applies to leather footwear in general.
http://www.danner.com/customer-service/boot-care/

Use your boot "gunk" treatment of choice, but it is important to treat the leather with something.