I'd echo the comments about Warriors 4-up. Unless you have one of the uprated weight variants (2400 lbs???) you're already at max weight with 4 adult males, half tanks and no baggage at all. And 3hrs sounds like an over-estimate of endurance to me. With proper reserves your half tanks will only take you 1.5 to 2hrs.
On top of that a max weight Warrior is not a relaxing environment to be in on take-off. You'll be willing that VSI to show just a little more optimism.
But hey, if you do your ferrying trips between Mull and Oban as described, and do your W&B and performance calcs properly, and you already have some grass experience, there's no reason why you shouldn't try Mull.
I have an Excel spreadsheet which does performance calcs for the Warrior I usually fly. I can send you a copy if you PM me on the strict understanding that you must put in the correct weights for your particular Warrior, which may be heavier than ours.