We can debate vague nationalities and undefined ethnicities 'til the cows come home.. the fact remains that, since dialogue is often at least half a performance, a performance's accent immediately becomes a defining feature of the character. The score?
TOS: one British accent, one Russian accent
ENT: one British (specifically, modern BBC-style) accent
Conclusion: a step back from TOS, especially when you consider what a Russian accent meant in the 60s.
Now, going back to the OP, Tahir wouldn't have had to do an accent; hell, the character could have been an eighth-generation Vermonter. Just a Middle-Eastern-looking face in the captain's chair would have spoken volumes about Trekkian progressivism.