This link compares the ring I'm after (the LFR version anyway) to the ring I currently have (my second ring is Bloodclaw Band, which is good for my spec). As you can see, even the LFR version of Asgorathian Blood Seal is better than the Timeless ring.
Although the Haste increase is attractive, I currently have 29.57% Haste on me (without BotK or any buffs). Should I concentrate on other stats, or keep stacking Haste?
Protection you should keep stacking haste for sure. Stop at 50% haste and you can start reforging more into mastery if required or going for socket bonus for extra strength for threat/dps. However if you feel that your having issues staying alive Haste/Stamina gem might just give you those couple of seconds extra on encounters and shouldn't hurt your haste stacking too much.
With proper itemization you can get 21250 around 570 ilvl I know I'm currently running into an issue at 575 where I have too much haste and I'm not using haste rings and I only have a normal thoks. Edit I overlooked one thing, if you use a spark or other reforgable stat trinkets you can hit 21250 much earlier