It rounds up. Your 2.5 wait is actually 3 seconds, meaning that it's faster to do it manually. Combos don't give bonuses if the first attack misses anyway, so I sometimes have to do the lead-in attack more than once, which a macro won't do for you. If that and the slightly longer delay are okay with you, though, then have at it.

Edit: You might also feel like throwing a stun in the middle of the combination to stop something bad from happening, but it's unreliable anyway since the gladiator stun is part of the global cooldown.