I don't know about you but I have yet to see any warriors who gem full ArP top any lists on World of Logs. I see that SimCraft suggest going full ArP but both the SS and Rawr says go softcap with trinket. That to me is enough proof that SimCraft seems way off.
I use Landsoul's spreadsheet as my only theorycrafting tool. With my current setup, it's telling me it's better to gem full armor pen for a slight increase in dps over full strengh. The only Armor pen trinket I have is Grim Toll and since I'm already too high on Hit, it's not worth it for me to gem for soft cap and still use Grim Toll.
All this to say that it really depends on your current setup. Should I get a Mjollnir and I would regem back to soft cap right away.
Also, I don't know anything about SimCraft since I've never used it before, but if it's giving numbers that are comparable to other theorycrafting tools (be it the maximum dps possible or a similar BiS list), then it shouldn't be off by a lot and it would probably only be different on things like averaging trinket procs on the whole fight or some minor things like the actual SEP of Hit rating after cap or expertise VS Armor pen to get to high values.