Hey Brett! Welcome to the forum! I have had a Standard for about 2.5 years now and I have experimented with many dual amp and dual cab combinations. I use 10 main presets, and ALL of them use dual lo-res cabs.
Of these 10 patches, 7 have 1 amp and dual lo-res cabs. For these, I can fit 2 overdrives, compression, PEQ, 2 delays and several modulations.
The other 3 patches have 2 amp and dual lo-res cabs. For these, I can still fit 1 overdrive, compression, 1 delay and 2 modulations. Keep in mind that not all blocks take up the same amount of system resources. Amps take up the most, cabs are second. Overdrive and compression take up a bit of space. Modulation and delays take up very little resources. Filter blocks take almost nothing! If I took the overdrive and compression out of my dual amp signal chain, I could probably have room for 8-10 blocks of PEQ, Delays and Modulations.
Ultimately, I don't feel the need to use 2 amps on very many patches. It just depends on what I'm going for. In the classic rock realm for example, Keith Richards and SRV layered 1 clean amp and 1 dirty amp to achieve their tone. In the modern metal world for example, some guys like the low-mid bark of the Uberschall layered with the bite of the Peavey. And that's cool. But not always necessary. In fact, when playing live I prefer to have a less layered sound. I usually play with a keyboard, acoustic guitar, bass, drums and several vocalists. At a certain point, an overly complex tone just gets washed out in the mix.
With some other modellers, it was necessary to use dual amps otherwise they sounded 'thin' or lifeless. Not the case with the Axe-FX. It sounds full and rich even with "only" 1 amp and 1 cab.
Finally, as for running your Axe-FX thru a couple of Mesa cabs, I will tell you that I have gone direct to the board (FRFR) for all but the first 2 weeks or so that I owned the Axe-FX. Here's why: if go thru a power amp into a cabinet, your sound will always retain some of the characteristics of the power amp and cabinet (ie. a 4x12 cab will ALWAYS sound like a 4x12 but NEVER truly sound like a 1x12).
By going direct, you can select which cabinet you want. I have tones that have 1x10, 1x12, 2x12 and 4x12 cabinets. I can go from a Fender clean and 1x10 to an Uberschall and 4x12 with the click of a button.
The obvious caveat here is that the more control you have, the more complicated it is to set things up initially. You will have to spend way more time than if you keep things simple with just one external cab. And hey, you might be a 1-kind-of-cab plug-in-and-play guy. But if you really want tonal flexibility in your rig, then FRFR is the way to go. Yes, there is a learning curve but the payoff is awesome!
Finally, with regards to the Ultra vs Standard question, there are many threads on this forum that go in-depth into this issue. In hindsight, I personally probably would have gone with the Ultra if I knew then what I knew now. There are some functional things that I would use. You might not need them. For example, I also play bass, and the Multi-band compression would have been good to have. It would have been cool to use some of the synth stuff (sometimes) and with the Ultra, you have the ability to attach reverb onto your delay trails (I don't usually use reverb live, but having it on my delays would add a bit more depth without losing clarity). So.... it's mainly bells-and-whistles stuff that I don't have with my Standard. For straight ahead rock-and-roll tones, it's ALL there. There is no difference in the quality of the amps or cabs between the two.
Hope this helps! Cheers!
-AL