I doubt that anyone has measured the metabolism in just the sucker. If someone has, it probably wouldn't be through the traditional measurement of metabolic rate, oxygen consumption. It would more likely be through measurement of enzymatic activity of something like citrate synthase. This doesn't directly measure metabolic rate, but rather potential for metabolic activity. Really, these are closely correlated, but it is only directly comparable to other measurements of that same enzyme. I hope that helps direct some of your searching. I did a bit of looking around and didn't find anything. (Sorry if this is all stuff you already knew...)
Anyhow, if I would to guess, I would guess that metabolic rates would be comparable to slightly lower in suckers than in mantle since there is slightly less muscle per mass in suckers.