Beyond the whole lame-ness of "power level" conversations in general, I really do not think a legionaire veteran of the long war stands up to a primarch with any real scrutiny. Reading the HH books puts that into perspective as even incredibly powerful warriors and xenos are put down by the primarchs before they even see the strike that hits them.
Fulgrim v. Avatar of Khaine.
Sanginius v. Blood Thirster
etc.
Also, one must not forget that having spent XXXX amount of years in the eye does not mean XXXX amount of years in realspace. Similarly, not all legionaires of the traitor legions are from the heresy times: they have been recruiting, implanting geneseed, or taking stock from traitor CSM chapters as well. The Iron Warriors book puts that into perspective. Also, not all legionaires from that time have ascended to demon hood or have been granted gifts.
yeah thats what i said...experience is important, but it doesn't lead to a massive upgrade at power
Primarchs are not even humans, there pretty much demigods ....they have a completely different genetic code from regular astartes if i remember right?