I cannot answer who has tuning tools for the 2005 C5, I can say as far as MTI since I have had to re-tune several C5s that vendor tuned is I would not recommend them at all
As to tuning, what I hear is the tuning tools still do not have full tuning abilities and have bugs so if any indepth tuning is needed to heavy mods that the tune would not fully do what is properly required.
Being the C6 has new torque functions and is now done in a new controller (TCM) tuning tools may not even be able to fully tune trannie functions at this time.
To make it worse the 2006 has a new ECM, heading for new EPA OB-III requirements thus any tuning tool for a '05 will not work on '06 and '07 GM has more changes
If a C6 owner was to purchase a tuning product, they could email me just the software and their flash file and I could do a true custom tune if they also email me a Ease testrun recording, thus tune is all done via emails.