I think getting 100% versus medium power will require more time. Same goes for OPM, but to lesser degree. Most of time both side will be besieging something, side which suck enemy fort first will go to (major battle) break siege and force them to retreat. Rinse, repeat. If you suck one of at most 4 France forts you will get to juicy undefended centre, if France can't beat you in direct battle you are sitting on 50% WS. To get 100% WS you have to take all 4 forts will be more troublesome. Once you are controlling inner provinces of BBB, you have better chance for stackwipe, which would speed up things a little.
@Up Forts in Russia won't be that important like in HRE or France, but you can't neglect entirely. On eastern border of Lithuania/western of Russia they will play major role.
OT - prestige will be even more precious, due to fact increased garrisons. You will be unable to assault minor and his minor friends to get 10*(1+number_of_friends) prestige in matter of months for 50 ducats (reinforcements for mercs).