As always Satorri you think about the minutia the rest of us overlook. Im curious what your total observation will be. My experiments have lead to these comparisons between Blood and Frost tanking (I MT ICC as Blood, and MT heroics as Frost, gear is almost equivalent ~245 or greater in both sets [Hit and Expertise (soft)capped in both specs]):
Frost's burst threat via HB and BB provides an excellent opener, however its baseline crusing threat is considerablly lower. I find that in heroics Ill quickly switch to Blood for the final boss due to its higher baseline threat. Blood on the otherhand is far weaker when pulling trash or with add waves (Precious for instance I switch to Frost). In testing Ive noticed that Frost will provide close to 5k TPS (read off Omen) single target and close to that on multiple targets. Its advantage is there is very little rampup as you stated. But Ive noticed that even when cruising the threat tends to top off around there, with very few spikes.
Blood being my perfered spec for raiding provides me with a slower ramp up than Frost, however when in combination of Empowered Rune Weapon the spec and generate the necessary initial threat prior to the DPS getting their rotation running. With this set up the only threat issue I have found is with my Druid OT when he gets an early crit before my HS. Combinded with ERW I can hit ~14k TPS in the start up. Cruising Im floating at ~9k TPS with spikes popping up to 11k.
I will say that I originally switched (in Uldar) to Blood tanking from Frost because I needed better single target TPS, and even now I still havent observed any changes to that relationship. I will say that the burst/starting TPS of any raid boss relies on using ERW, regardless of spec, and while cruising using RP and Blood Tap to fill in gaps/save rotations is a standard.
Although I doubt this will differ your opinion on the observations, I just wanted to report some additional data for your study.