Hi community!    
If you have asked yourself which pitool version you should use for DCS 2.5.5,  
I may have a few answers for you.   
I use Pimax Artisan HMD for myself.(FIRMWARE: V2.1.255.258 )   
PC SPECS: 
9900KS@5100MHZ/ASUS 2080TI-OC-11GB@2055MHZ/32GB DDR4-4000-GSkill/ 
          SAMSUNG-970PRO.M2/AORUS Z390 MASTER/ WATERCOOLED /   
I tested following versions with exact same settings.  
Every time clean install(remove 6 folders, CC-Cleaner/restart,install pitool,restart)   
PITOOL:  
120HZ / NORMAL FOV / Render quality 1 / 
/Smart smoothing OFF / FFR OFF / COMPATIBLE WITH PARALLEL PROJECTIONS OFF / 
/COMPATIBLE WITH VIVE ONLY OFF / HIDDEN AREA MASKS OFF/   
STEAMVR: 
65% / MOTION SMOOTH OFF / CLASSIC MOTION SMOOTH OFF /   
DCS 2.5.5: 
PD: 1.2 / 2x MSAA / PRESET HIGH, MIRRORS ON / SHODDOWS FLAT / ALL EFFECTS OFF, ONLY RAINDROPS AND WHIRLWIND ON/     
ALL TESTS WITH SU-25 -> MISSION: 4th from above(GUNS AND UNFEED ROCKETS)first 40 Seconds on airstrip straight looking forward. 
 
RESULTS:   
PITOOL: 1.01.249: 45-46 FPS   
PITOOL: 1.01.253: 48-49 FPS -> FOR ME THE BEST PERFORMANCE SO FAR. SOMETIMES IT FLICKS   
PITOOL: 1.01.254a: 50 -49 FPS ->BEST PERFORMANCE BUT VERY FLICKERING WHEN MOVING   
PITOOL: 1.01.255: 40-41 FPS -> LITTLE TO NON FLICKERING   
PITOOL: 1.01.258: 39-40 FPS - NO FLICKERING @ALL BUT LOWER FRAMERATES.  
I think the picture is a little bit sharper and smoother but @ this point my eyes are bleeding and it could be a delusion :D     
I hope you can do something with this information :)    
BR Henning