Well, the initial question was about romans being able to be on top, that includes off, pop and raider. Pop and off are also supported by income from raiding, hence the raiding debate...
A roman defender will NEVER be able to be a top raider, so that issue can be dismissed right away

Also it has been shown that even phalanx can raid, now since praets have better attack than phalanx, of course that praets can raid too. How well they do, it is an entirely different matter. To end that subject right now, any unit that can carry bounty can, of course, raid.
Again, back to original question. At the start of the game, there is no way for romans to compete with fellow teutons. After they tech to EI, they will start to catch up a little bit, but by that time teutons will already have considerable lead. Also EIs are expensive. If they keep their cool and don't mess their troops in reckless battles, starting with midgame they can make it to top raiders. This my own opinion, with the assumption that those 10 top raiders have similar skills, in other words when raiding efficiency doesn't come mostly from the player's skill, but from the units stats themselves( I mean when the units stats is the limiting factor - this includes cost, training time, crop consumption, attack, defense etc.). Hope this is more clear now.
Cheers