I agree with your analysis of GR3. And even as his efficiency became worse as a sophomore, he was still a much better player than Irvin has been this year.
It's tough to really draw any conclusions from Mathews' play last year, as he was stuck in a guard rotation behind Murray, Ulis, and Briscoe, all of whom were five star recruits. I don't see Zak Irvin cracking that lineup, either.
I don't think we're going to need to rely on Mathews to be a passer or a consistent three point threat. I see him as a slasher, and someone who can create his own offense at the rim off the dribble. I was looking last night, and in 2011/12 Tim Hardaway averaged nearly 15 a game shooting only 28% from three. That's not real efficient (he shot .419 overall, which is also not great), but he was still a good scorer for us.
The problem with Zak right now is, for the most part, he can't do anything out there. He's throwing the ball away passing it, he dribbles it out of bounds, his shot is way off, he gets blocked going to the rim - it's generally a disaster. To be honest, in most games he's a net negative on offense. Mathews will probably have limitations, but I do at least expect him to be able to finish at the rim, and be the beneficiary of residual looks.
I also have a feeling Simpson is going to be really adept at creating looks for others. I see it already. He's a step quicker than Walton, and he's finally getting comfortable. What that has meant is that in the pick and roll, he's been able to either finish with a layup, or if help comes, find an open guy. I think his continued ability to do that will be huge for Mathews.