So on spine of deathwing, i have noticed my damage seems a bit low on the tendons compared to the rest of my raid if i use dots on the Tendons. Also with 3 391 tier pieces is it worth it to switch to t13 397 gear?
If I sim your character for the T13 2pc bonus, I get a 93 dps decrease. Therefore, if the piece you upgrade is worth more than 93 dps, then it's worth changing, otherwise no.
As for the tendon, I think dotting is still better. With a good usage of CDs you should do well.
Something that is helping us is setting a Main assist. The issue with dots is you need the time so the shorter the duration your on the tendon the worse they become. By setting a main assist you can target that players target ( he should be using a macro to focus burning tendon as soon as it pops) and start blowing it up. My dps was low because i was losing 4-5 sec before i was actually getting on the tendon(by clicking on nameplates) this frees up those GCD's so you get your dots up then start blowing up the tendon.
Using a main assist is a piss poor way of DPSing. Don't rely on anyone else to "carry" you. Use "V bars" (enemy nameplates) and be more aware. As soon as you can target the tendon, do so. Don't wait for someone else.