I'm toying with this idea...
Run a 16 tooth (which is already installed) front sprocket and a 45 tooth rear sprocket and change my tires to a 40/60 (40% road/60% dirt) set up. If you ride mostly on the road wouldn't this set up give you a relaxed cruising speed and smoother ride?
Anyone tried a set up like this?

I run 16/45 and Shinko 241's which I consider 50/50 tires. They're good in dry conditions, be that street or dirt. They're not so great on wet asphalt or significant mud.

How does this gearing combination effect shift points and top end speed - cruising?

At any given road speed, the engine rpm will be 6.7% lower with a 16 tooth front sprocket than a 15. The engine will also send 6.7% less torque to the rear tire at any speed. I find it helps make the bike a little happier on the street. It has a minimal effect on top speed. If anything, the bike will go a fraction slower due to the reduced torque and rpm. Engine rpm at a cruising speed of about 55mph will be about 400 rpm lower.

This is a good tool to see the effect of gearing changes... https://www.gearingcommander.com/

I think tire choice should be based on what you intend to use the bike for. I already tried the 16 tooth front sprocket, and it didn't work for me. The engine simply does not have enough power to pull a higher gear. It lugs the engine easier in all gears, feels sluggish, and has a slower top speed.

Everyone who's tried a 16 tooth front sprocket has an opinion on whether it's worthwhile or not. From personal experience I'm with Jerry on this. The little 223cc engine the XT225 uses "simply does not have enough power to (effectively) pull a higher gear."

On the other side of the equation, going from the Yamaha's stock 15 tooth front sprocket to a 14 tooth, I ended up with a 1st gear I almost never used and a reduced top speed that was like throwing a speed limiter into the works. In some situations, namely riding rugged desert washes where finding a way through broken stream beds and loose rock slopes was a real chore, the 14 tooth made a name for itself. Otherwise, in everyday use, it was more hindrance than help.

When I went back to the stock 15 tooth sprocket I realized Yamaha had precisely hit the 'sweet spot' with their out of the box gearing for the XT225, and after that I left it alone.

I've been running 16/47 sprockets, so slightly higher than stock. I also run a 120/90 rear tyre, this can make quite a difference to the gearing. This set up is slightly over-geared.

People talk about the sprockets without saying what tyre size they use. My Brazil model (stolen) had stock 15/45 with a 110/80 tyre, that was under-geared for the road but good on the rougher trails.

I'm just changing the front to try 15/47, this with the 120/90 tyre should be a happy medium for road/trail use.....for me.

The stock 15/45 & 120link is, according to Gearing commander, a bad mix for wear.

What size tyre you running? A 16/45 might be ok with a 120/80 tyre, not tried that mix.

A 16T sprocket is cheap enough to give it a try, swap back if you don't like it.

My 2006 XT225 is running a 16 tooth countershaft sprocket, with stock size front, and a 4.10/18 rear. Both are Shinko 244 Golden Boy Trail tires. I can't imagine a better combination. On pavement, the engine is running at a comfortable RPM at 65-70 mph, and the tires do not squirm during fairly hard cornering. On dirt roads, the tread of the 244 tires provides good grip on loose gravel and mud, and I almost never have to use first gear.

I almost never ride my XT at over 60 mph, usually closer to 55 mph on the road. About the only time I ride it on the pavement is to get to a place where I can ride it either off road or on dirt roads/trails. I have several street bikes for road use. IMO, as much as I love the XT225, it is not suitable as an only bike if you plan on doing a lot of road riding. The engine is too small to hold highway speeds, and the seat is almost unbearable on pavement. It does make a fine around town bike, especially if you install a luggage rack.

I don't think there is any "only" bike, although I can imagine a time in the not-too-distant future when the XT225 will be the last bike in my garage. I have a BMW F700GS (~460 pounds), which is much easier to handle than its predecessor, a 1999 R1100RT (~640 pounds), but the XT225 is so light and flickable that it is — by far — the most fun of the three. I just wouldn't want to ride it more than 100 miles in day, even on a modified seat. Compared with the XT, the GS too tall tall and heavy for dirt for riding someone my size (5' 6" ~145 pounds). The RT is great in cold weather and for long interstate rides, but it is a handful at low speeds, especially 2-up.

If by "highway speeds" you mean 4-lane, divided highway, I agree, but with a 16 tooth countershaft sprocket, an XT225 should easily hold 70 at ~6500 rpm. Above 75 the frame/tires/suspension become a little twitchy, but power is not the limiting factor. For mountain roads, it has enough power to handle anything I have encountered between 1500 and 3000 feet. With a carburetor, I would expect it to start gasping over 5000 feet.