pretty hard question to answer actually. Gas rifles are never as consistant as a spring rifle. You may buy two of the same rifles (asuming they are both gas) and one might get 80 yards and the other won't be good past 30 without tweeking it. Had that problem with a couple KJWs. Tanakas are higher grade than KJWs so this may not be a big issue, however I have owned two VFC M82s. One was an absolute tack driver at 100 yds and the other couldn't hit the side of a barn at 20. If you want consistancy, just go spring.
ive been doing some searching on the net, and found a conversion kit to CO2 at www.magnumbb.com, its their coldshot system. read some good reviews- its dual regulated, so its very consistant. couple that with a G & G power pack, a 2roy hop up, and a good tightbore... rave reviews with this combo. shooting around 450-500fps at 100psi, and good groups out to 75-90 yards.