Possibly, but lets consider what Kensai III adds that a 12 fighter 6 ranger 2 x can already get... Kensai II and Tempest I... A finesse Elf or Drow (rare builds I know) is going to out dex the Helf, a Strength Horc is going to out STR the Helf, a Human gets free Khopesh feat...
Helfs are stuck in the middle with nothing much to recommend them as a melee class.
Now Compare a Helf FB III tempest I with a Horc FB III Does Tempest outweigh all those Strength and two handed fighting AND power attack enhancements? The Helf also is using almost all AP's for the PrE's doubtful he'll have any left even if he did have a PA enhancement line (which he doesn't obviously)...
IMO on the face of it it sounds OP but when you consider a Horc FB III is still going to be out DPS'ing the Helf it kinda puts it in perspective. the neat part is it actually encourages deeper multiclassing... For example a Helf "battle mage" build might take the first 3 arch mage PrE's and still be able to take a fourth tier at 15th level, even with 6 melee levels (a 6 ranger 14 wiz build or some flavor of that). So you could have a sort of "Arcane Knight" build flavor... It wouldn't be uber but it would make the trade off a little easier to accept. And make the deep multi a little less gimpy