So caved in and got Ice Brand for my RDM. Only done the easy tests, the additional effect: Ice Damage is 100% procrate and was doing 168-186 in both offhand and main hand on Apex Toads. Orpheus Sash seems to add to damage. Your own enspells will write over the swords effect.
Edit: Another quick test, killed about 25 Apex Jagils/Toads with just auto-attacks and Sanguine Blade. I was getting the same damage with Naegling/Ice Brand and Ice Brand/Naegling at 1000-1200 TP. 11953 on Toads and 16568 on Jagils. By same numbers I mean exact same numbers with same WS set.
I also tried Red Lotus Blade on around 20 Toads/Jagils with the same setup and was also getting the same numbers at 3000% TP. 12960 on Toads and 16920 on Jagils. So it appears that the WS INT+10% might work off hand as well.