" to be perfectly honest...
0.25" "groups" consistently at 100 yards would IMO, be tough with most any 17 caliber rifle ( centerfires included ) unless you are calling 2-3 shots a "group"
I have an old Marlin bolt action 22, that I converted to 17 Mach 2 ( a 22 LR case necked down to 17 caliber ) with a barrel liner, & chamber reamer... 0.5" to 0.75" - 5 shot groups are not uncommon at 100 yards with it, in fact I shot a pidgin off the silo from approximately 100 yards, with one shot to the head... one thing the OP mentioned was subsonic, & that just isn't going to be possible with a 17 caliber firearm ( bb or pellet gun yes, but the Mach 2 is the smallest of the 17 caliber cartridges, & it's 3 times the speed of sound )... that speed is what adds to it's killing power... I use my Mach 2 for farm yard varmints, as even the slightest of twigs, grass, or weed, & the bullet self destructs, never a ricochet... we use ours a lot on coons the outside dog trees, & one well placed shot is all it takes for easy dispatch of animals as big as a large coon
if you are looking for subsonic, I think you need to go to a heavier bullet, in a larger caliber... try match loads in 22 LR
BTW, just so you know... my 17 caliber experience includes... 17 Mack 2, 17 K Hornet, 17 Fireball, & 17 Remington