Flexible gravel/road bike - 38c to 25c - Will I really notice the difference?

G-Ones Allround use the same compound as Pro Ones and their casing is very thin and supple. They roll at least as fast as half decent road tyres on smooth tarmac and even faster on the rough stuff. They certainly didn't prevent me from getting from Southampton to Cornwall in a day (with a tent and some camping gear).

Considering that 25mm is a choice of pros, logically a typical heavier, slower and less aero non-competing weekend warrior / sportive rider shouldn't even consider anything narrower than 28-30mm (on wide rims) for "fast" riding. It's a no brainer really. One size doesn't fit all.

Pro Ones are lovely but they can cut badly. G-Ones Speed 30mm are a more robust alternative. Another option would be Specialized S-work Turbos (tubeless) in 28mm.

Wheels will make almost no difference to the ACTUAL (not perceived) speed.

Why can't you just swap your tyres over, why even bother over maybe a couple hundred grams if that on the new wheels? Spend some of the saved money on a tight top and an eneme, spend the rest on gels .

On non-aero wheels, I notice about a 1-2 mph difference on a tarmac ride between my road bike and gravel bike on a 40-60 mile ride. That's moving from tubeless 42mm Maxxis Ramblers to 28mm Conti Grand Prix GTs. When my aero wheels go onto my road bike with 26mm S-Works Turbos, this becomes more like 3mph+.

If you are going to do it for the sake of additional speed, then go for soemthing with at least a mid-deep section, as that is where you will see gains.

G Ones are very fast.  I've done 100 mile Sportives with them and club runs.  I can cycle over 21 mph on the flat with them.  Don't change them, they're great and comfy.  Only downside they wear quite quickly.  25mm will be only a tad faster with less smooth ride.  I only go to narrow tyres if I'm wanting top riding speeds.

I have a CX bike with 35mm CX tyres and it's an absolute pig to ride on tarmac. On  my road bikes a change of tyres (from 25mm gatorskins to 27mm open pavé) made a noticeable difference to speed (higher), comfort (greater) etc. over short as well as long distances. In your position i'd first try a new set of smaller fast-rolling road tyres, say 28mm, before spending on new wheels. If you like the tyres but still feel the need for new wheels then you haven't really lost out, but you might not want/need new wheels.

done something similar but I'm at the getting away from the broom wagon end ... had two sets of wheels for my cx stylee: heavyweight about 1kg more running 32mm marathon swalbes on generic commuter wheels - puncture proof but add weight v 25mm lightish road specific on around 1500g factory wheelset - reckon for me and no athlete about 2km/hr difference - kept careful check on chain wear so could swop with no worries- didn't have any problems with caliper position but probably luck with hubs - more recently just running on 35mm swalbe pro ones and reckon with wheels in area of 1500g do the business as in lighter wheelset  makes up for wider tyres -and  seems like tyre technology has moved on a lot so depends what calender brings not so sure  - to conclude if go for it issues are chain wear, caliper position and if tubeless remembering to rotate the spare wheel set? 

With 25mm tyres you will be able to accelerate more rapidly and climb more rapidly. Cruising speed will probably not change very much (main issue here is aerodymanic drag). However you will be slower over poor surfaces and you will definitely notice a difference in comfort.

I have two bikes, fairly similar spec, one with 25mm tyres and one with 32mm tyres (OK, the 32mm one is a bit heavier). On a typical ride there is ~1% difference in average speed, but there will be sections where one is significantly faster than the other, depending on surface and gradient. For you the difference will probably be a bit greater, as the difference in tyre size is greater. But don't expect miracles. 

Yes. You will be significantly faster. Depending on the road surfaces I'm not sure how much you will notice the comfort difference. It's not like you're floating around on a cloud of eiderdown. 

However, you will make even more gains with a more aero position.....

You will be faster but will notice the reduction in comfort more.  

But you won't be 5mph faster everywhere like the marketing would have you believe. 

I would try a big ride with your present set up and make your descision based on that experience.