there's still some room for improvement.
Really? I mean, I guess there is always room for improvement, but, I'm not sure Tokina is gonna be the one to do that.
The 70-200 f/4L was considered a gold standard for sharpness until Canon released their last two f/2.8 zooms. Go to any review site
and it's basically got perfect marks
The only thing Tokina can really improve is matching Canon's 5-stop hybrid IS, or by making it a lot cheaper (<$1000 retail).