After running a 20 rogue mech, the capstone is not worth what a splash can bring. Rogue 18 is all you need.
So the real question is: what to splash?
2 arti is the way I lean. You get rune arm, spells (conjure bolts, enchant weapon), rapid reload, all xbow proficiency, dog for doors/levers, spell points based on your main stat int, and possible extra synergy after arti trees updated.
If you can optimize rune arm use, this is going to be best dps. But the truth is ranged characters kite mobs and rune arms slow you, but you do get some dps advantage even when not optimal and fireball types are great for door bashing. Conjure bolts scrolls are fixed, so that’s not a big deal. Xbow prof only matters during leveling. Spell points are very important for epic destiny use.
1wiz/1ftr is another strong option. Still get int based spell points, invis sla, a meta magic, spells (shield, jump), skeleton for doors/levers, fighter bonus feat, fighter profs.
Invis sla, enough said. Free meta, probably quicken for EDs and sla. Skeleton takes 3ap so a small loss over free arti dog. Fighter bonus feat is a wash since you lose rapid reload. Heavy armor proficiency is nice, I like the option to run high prr/mrr and swap for evasion in certain quests.
There are other viable options, but these are my top contenders currently.