PYRO T6 Improved Ground of Power States that "the damage bonus while standing in PYRO R6 Ground of Power, is increased by an additional 5%". While GoP states that "while standing in GoP the Mages 'fire' damage is increased by 10%." Reading this strikes my curiosity as perhaps, the additional 5% from the talent is in effect for not only fire damage, but for all spell damage. The description of IGoP is not specific to this. Perhaps, this was not intended but nevertheless is how the mechanic works?
I think you're reading too much into it.
Ground of Power increases the mage's fire damage by 10% and causes them to take 10% more incoming damage
Improved Ground of Power increases the additional damage from Ground of Power by 5% when fully invested, since it's referring to the additional damage from Ground of Power it's talking about fire damage since that's what Ground of Power increases