You won't rub or have much power lost, you may not even really notice it unless your constantly flooring it from a stop. However, are you mailny on the trail or do you plan on trying to get through big mud holes? I had Zillas ordered from 2 places online and they were out, thankfully. I started watching videos on Zillas and was not impressed with their mud performance. I do have to say they are the lightest trail/mud tire and will NOT impact your performance on the low end like other 28s will. I ended going with the 30 inch Monster Mayhems (Midweight Tire) since they are agressive in the mud but can still handle well on the trail, if I was going to go 28s I would've purchased the 28 inch mega mayhems, they are SEXY with a busier pattern and ride smoother than the Monsters on the trail, also they come in a wider size.
How long have you had your Viking? I was in the same mindset when I bought mine, I was dead set on getting 28 inch Zillas so I didn't have to do the clutch work but here I am less than 2 weeks later with 30inch Monster Mayhems and a JBS clutch kit on the way!