Aerobic respiration of glucose (the primary source of energy for animal cells) proceeds according to the following formula:
6O2 + C6H12O6 -> 6H2O + 6CO2
We can approximate the energy gained in this reaction by looking at the heat of formation of each of the participating molecules; the energy gained is the difference between the total energies on each side of the reaction.
Heats of formation are as follows (all values taken from NIST Web Book):
C6H12O6 174 kJ/mol
H2O -285.83 kJ/mol
CO2 -393.52 kJ/mol
Which results in the following totals:
6*0 + 174 = 174 -> 6*(-285.83) + 6*(-393.52) = 4076.1
for a net energy gain of 4250.1 kJ/mol of glucose.
If we directly replace O2 with SO2 in the above reaction, we get
6SO2 + C6H12O6 -> 6H2O + 6CO2 + 6S
Elemental sulfur has a heat of formation of 0, just like diatomic oxygen, so the only change in energetics comes from the non-zero heat of formation of SO2, which is -296.84 kJ/mol. Since there are six of those molecules involved, that takes 1781.04 kJ out of our net energy gain, leaving us with 2469.06 kJ/mol of glucose- a little over 60% of what we get by using oxygen.
Other reaction products are possible, including H2S, CS2, and COS, with the following heats of formation:
H2S -20.50 kJ/mol
CS2 89.41 kJ/mol
COS -139.0 kJ/mol
so you can try fiddling around with different ratios of reaction products, but I don't think you'll get better results than just producing elemental sulfur as a byproduct.
So, if nothing else changes about these creature's metabolism, the kinds of energy storage molecules they use, and so forth, an SO2-breathing creature would need to eat about 66% more food and breathe 66% more air to maintain the same power levels as an equivalent Earth creature. Given the enormous variability in food intake and metabolic rates among existing large animals on Earth, it does not seem at all implausible to me that you could have a large, energetic creature breathing SO2 under those constraints. I could certainly survive just fine having to eat twice as much food and breathe twice as quickly as I do now--and birds, for example, have much more efficient respiration than humans, so were I properly adapted for this environment, with more efficient lungs, I could probably get by just fine merely by altering my diet to increase the ratio of starch and sugars and only eating a smidge more than I have to as a normal, O2-breathing human.