Stats are random, so while the breastplate forged with the dragon heart gem is potentially better, it is not guaranteed to be so. Presumably that is why you have the option to buy back the original, if you wish. There is no value to the dragon heart armour over and above what is listed in the stats, except to show the dwarven king (to get the helm) and to wear once as part of the full set (to complete the associated quest).
If you quicksave after getting the upgraded armour, you can check the stats and then quickload to see if you can get something better. Save the game after finding good candidates, if you want to keep reloading (for objects placed directly in your inventory, you need to move them in order to set their stats; just identifying them is not enough).
Grisha can not upgrade any other armour, just the breastplate.
FWIW, this is what I got, reloading for both:
Breastplate Of The Dragon
Durability 30
Charm Quality 1
Armour 390
6 Strength
6 Intelligence
60 Vitality
100 Magic
15 Resistance to Lightning
15 Resistance to Poison
2 Hearing
Heaven's gift +1
Breastplate Of The Dragonheart
Durability 12
Charm Quality 3
Armour 440
2 Defense
100 Vitality
60 Magic
15 Resistance to Fire
40 Resistance to Lightning
15 Resistance to Spirit
4 Hearing
Survivors Instinct +1
Nice stats, especially the armor #. the Durability of 12 can be a problem though,
especially for weapons: my "Sword of the Gods" is DUR 12, and I am always needing to Repair it!
btw: what exactly is benefit of "+4 HEARING" ?? + X SIGHT"??
another question: my sword has "FROST(4)" does that mean 4 damage from Frost, or it casts FROST, at a level 4 skill??
