Conti 4 Seasons 28mm measure 26mm on Open Pro rims.

I'm thinking about switching from 23c to 25c does it make that much of a noticeable difference?

I think it depends on the tire, as you have noticed some tires measure differently. With that said, 25s will make some difference because you can run a bit less pressure. I currently commute on 23s but will be switching to 25s when the time comes.

I got about 4k miles out of a set of Continental GP 4000s. Still had a fair amount of rubber left on them, but the sidewalls were going. Minimal flats, and handled nicely in the rain. Replaced them with a set of Gatorskins, and so far so good. Haven't had enough experience in the rain to pass judgement on them in that regard, but I'm generally pleased. Went from 23mm to 25mm with the Gatorskins, and it is a bit more cushioned, but I haven't noticed any performance decrease with the switch.

Briefly ran a set of Continental Touring Plus in 28. Very comfortable, but heavy and slow. (Ran nice on fire roads though.) Worst luck of any tires as far as flats go with those though.

I commute on gatorskins and they won't grip as well in the wet as the 4000s but mine have lasted for what seems like forever so I assume the rubber is harder.

I found the Gatorskins to be rubbish in the rain, and switched to 4 Seasons and have been extremely pleased with them. Just put on my fourth set today.

i've rode nothing but conti 4000 25's for about 10yrs, love 'em (except for big apples on my stokemonkey). never had a flat that wasn't due to low-pressure/pinch flat.

but for my new commuter i want cushy and i've got a pair of marathon supreme 42's (haven't built up the bike/rode them yet).

If the 25mm tire measures the same as a 23mm tire@23mm, can I still run a little lower pressure? Doesn't make sense to me.
I'm trying it though.
I used to run 100psi on the 4000s 23mm tires
Trying 95 psi on the GP 4 seasons. Feels a little softer but the tire is still new. Might go further down to 90psi

I only weigh 146 lbs and my backpack is under 5 lbs, so, I don't think I need to pump up the tires more.
Only time will tell.

I don't set pressure according to the size printed on the tire, I measure width with a caliper and set pressure accordingly. A true 25 can be run at a lower pressure than a tire that measures 23mm.

Not only this,but the rim width will also make a difference. My '05 BBU has 15mm rims and came stock with 23mm Hutchinson Flashes. My '09 BBU has 24mm rims and 28mm Vittoria Randos. I swapped the 23's for 25mm Specialized All Conditions,and thought they looked kind of big afterward. Checked with my caliper,and the 25's actually measure about 27mm,while the 28's on the other bike actually measure about 26mm.

So I'm riding a bike with 25's that are almost 28's,and a bike with 28's that are almost 25's.