hmm ok, then mine with 129 (unblessed) is slightly stam bugged?
Yes. For a time, stat effecting spells used during taming could effect the pets stats (up or down). I believe that has been patched for a while now. My first greater dragon was also stamina bugged, but instead of having more than 125 Stamina (@125 Dex) it had 119 Stamina (@125 Dex).
Small stamina increases (from having bugged stamina) does little for the pet. It gives it some protection against weak casters using Clumsy or Curse, since the pet can maintain its maximum swing speed (achieved at 120 Stamina, swing speed improves by 0.25s for every 30 Stamina (same for players and pets)). However, this benefit is lessened since the greater dragon is also a caster and will Bless itself (offsetting the effects). There is no benefit to the swing speed of the pet, as you pointed out earlier, since 150 Stamina is required to swing faster. It will not get to 150 Stamina when Blessed. The dragon would receive +13% of its Dexterity from Bless (@120.0 Eval Int), which equates to +16 Dex/Stamina (125 x 1.13 = 16.25). A pet would need a Dex/Stamina of 133 each to reach 150 Stamina, when Blessed.
However, the Bard Song Invigorate, coupled with Bless should allow the OP's dragon to get to the 150 Stamina swing speed. Invigorate gives between +8 to +14 in each stat (based on the bard's real skills), so the dragon would get that bonus to it's Dex/Stamina. I have not tested it (my tamer does not have Provoke). See here for more info on Invigorate:
http://stratics.com/community/threads/bard-faq-a-must-read-for-all-new-bards-updated-2012.168520/
The OP has a nice Dread Mare. Best of luck on the sale.
Stayin Alive,
BG