What I've noticed so far is less racking.
I say this tongue-in-cheek, but it's literally true. In the past I racked every 3 months, which unnecessarily exposes the wine to air, and causes loss of volume as racking has some wastage. Sur lie is an easy technique -- ignore the wine. Bâttonage is easy in the barrel, since at topup time the volume is reduced so I can stir gently with a drill-mounted stirring rod, then top up. This also distributes the oak flavor from the oak cubes (barrels are neutral so I add cubes), making the taste testing more accurate.
In the carboy? I'd withdraw a large sample using a thief, stir the wine, and then replace the sample.
I've been happy with the wines I've made, but without something to compare against, I can't say for sure what difference it's made. That's why I would like to do a test. I've considering purchasing another FWK Tavola for a quicker drinker (no skin packs, minimal oak), and moving that into a pair of 3 gallon carboys will let me do the test.
It appears Costco sells kits only Canada -- I searched and it came up Costco.ca, and kits don't appear for my local (NC) locations. I don't know anything about the quality of the Costco kits, so I can't comment.
I'm not in favor of shorting water in kits. I've had a few kits in recent years where the acid was out of balance after I shorted the water -- these were WE or RJS kits. Given the results I got with the new WE formulation, I'd go with a WE kit, and add raisins, currents, or something similar. Maybe a sliced banana or 2 for body. Add to the kit, not take away.