I'll jump in and cast my vote for keeping the audible Vader reactions. As I see it, the issue isn't so much Vader's strength as much as it is Luke's. Everything is leading up to Luke's total defeat and humiliation. Yoda insists that his training is incomplete and his strength is altogether too underdeveloped to face off with the Dark Lord. Vader is making all the rules, calling all the shots, and holding all the cards. It's his ground, his turf, and his terms. "All too easy."

And yet, all odds to the contrary, Luke shocks both his would-be captor and the audience by matching ruthlessness with passion, cruel intention with heroic bravery. He fights with strength unequal to his training, and Vader is utterly taken aback by the aspirant's resilience. Even when he has forced Luke to yield ground (not unlike his duel with Obi-Wan, by the way), and seems to have regained the upper hand, Luke forces Vader's defenses aside and deals a lightsaber blow to his shoulder. His reaction is a mixture of shock, pain, admiration, and anger, all of which rise from his charred lungs and pass through his vocal modulator as a muffled outcry.

It's an interesting dynamic. Even though Vader goes on to overpower Luke, he ultimately fails his mission, and the would-be Jedi escapes with his friends and ship intact. The Empire struck back, but Vader's efforts have ultimately been thoroughly and utterly frustrated. What's a Sith Lord to do to regain his plunging self-confidence? I guess he could always go build another Death Star...