25yrds is an excellent distance, no worries. I'd get the JSB 13.43gr pellets, which aside from being possibly being the best pellet for that gun, should put it ~900fps which is a great speed for accuracy. From 900 on up to super sonic it gets worse, and while I haven't really had issues with going to 1000fps, 900 is technically better. Plus heavier pellets generally shoot better anyway, especially at longer ranges and they tame the gun a bit which makes for better shooting all-around. So the 10.34gr JSB is what's I'd consider the absolute min weight, which is likely ~1000fps and thus costing you at least some accuracy. JSB also makes a 16.something you might want to try just to see what happens. It'll probably shoot just fine and be between 750-800fps, but at some point going up in weight the gun will have issues and I don't know where that line is. I think it's higher than 16gr, just saying I don't know for sure. What'll happen is velocity will become erratic, but if the gun is ok with them you might like 'em.
I mention JSB mostly b/c I consider them awesome pellets and they aren't very expensive, plus that 13gr is a unique weight and is imo ideal for magnum springers like the XL, Diana 350 etc. I think the H&N Barracuda Match are awesome too, maybe even better but they're only in 10.5gr. The Crosman Premier round nose are great as well, also 10.5gr. I'd try 'em all b/c often/usually a chinese gun will shoot one much better than any other but my $ is on that 13gr.
You can also try getting a 22 barrel for it b/c the gun works much better in 22, and makes more power which I assume you want since you bought an XL. A barrel should be ~$32CA from the Gravel Agency.
Aside from that there are many other things that can hurt accuracy in that gun. The barrel drooping downward, which you can see, or use a yard/meter stick to verify, can cause a couple issues with the scope. The main seal is poorly designed and imo all XL owners should get this guy:
http://tinyurl.com/o7jsmwz It not only makes the gun shoot smoother but also nets a good chunk of free power. The breech seal often leaks so check that and maybe replace with a rubber O-ring. The barrel has serious flaws which you can verify by pushing a pellet thru by hand with a dowel or cleaning rod. You'll feel it's both rough and more importantly has tight spots. The muzzle end may also have damage anywhere from the very end to several mm down. The barrels side pivot washers are just plastic so they offer little support and the barrel can move side to side, which you can verify. The barrels pivot bolt is not precision (lumpy), and since the barrel uses that to settle on it can cause it to point in diff directions, especially if the side washers are weak as mentioned.
Things I do: Shim the scope so its angle matches the barrel. Replace the plastic barrel washers with metal ones that are custom fit. Sand the barrel bolt more round. Sand the barrel to smooth it and remove tight spots, and if needed cut off the bad end. Replace the seals as mentioned, plus I do misc other things. A lot of this stuff is tricky and/or you need to know critical info before doing, but I made a how-to guide for it, pix and everything. chevota at hotmail and I'll send it to you. If you buy a barrel let me know first b/c there's some things you need to know so you can decide on what other parts (if any) you need. Meanwhile get those 13gr JSB's!