I've been using Remington .38 gr. hp Subsonic ammo and not getting good results. I did some online research and found that CCI makes a .40 gr. FMJ in the .22 caliber. anyone have experience with this ammo.

In general which ammo will kill an animal more efficiently under supersonic speed a HP or FMJ?

I've had good luck with using their 32gr Stinger Rounds.
I pick them up in bulk at Cabelas. 500 rounds for $69.99.
Some info below

CCI’s Stinger and Segmented Hollow Point .22 LR ammo launch their rapidly expanding 32-grain bullets at a blistering 1,640 fps. Stingers are loaded with a traditional hollow-point bullet, while the Segmented Hollow Points feature pre-fragmented bullets that separate into three equal pieces upon impact to create three devastating wound channels. This special bulk pricing makes honing your skills more cost effective than ever.

Knocked out a Coyote the other day with Stingers. I think you can get them at Wal-Mart for $6.99 for 50. If you're wanting to kill something with a .22LR then you probably won't find a better ammo.

By the way, in answer to your question about hollow vs. Full metal jacket. If a hollow point .22 Stinger won't kill it then you probably need a bigger caliber rifle. All a FMJ is gonna do it penetrate deeper. It doesn't have a whole lot of shocking power because it lacks the expansion.

i always have good luck with the regular old reminton 22 lr . nothing special , the fmj is just gonna go through an not tear up alot , the hollow points will tear up fleash but say you shoot a hog in the head an hit bone it shatters the bone but dnt go through , soild lead bullet does the best i think, i have killed atleast 10 bobcats with the cci 22 CB bullets , all head shots ,an killed pleanty of hogs with 22 lr soild lead, what ever your shooting hit it in the head an it will drop , makes better markmenship too