The T-800 can be outfitted with the tissue, or it can be bare. The "skeletons" that John shoots in the future is still a t-800. If it is outfitted with skin to look like a human, it is assigned a model number -
"However, the T-800 Series also has different configurations of living tissue, each one giving it the appearance of a different person. The physical appearance of the living tissue (which was initially based on photographic records from Skynet's databases and laser scans of human soldiers) is therefore also given a Model number."
Though, if put into direct battle, it doesn't need to infiltrate. It just needs to fight:
"T-800's being deployed straight onto the battlefield as soldiers do not need to be disguised and so do not receive a biological covering."
The difference between the T-800 and the T-850:
"The 200 kilo T-850 Series endoskeleton has been upgraded from the T-800 Series in many ways, though these are not necessarily visible from the outside. The main modification to the T-850 Series is its increased titanium alloy armour, hardening it to Plasma weapons, which the Resistance has been able to acquire more of in the last few years. Tougher and more resilient, the T-850 Series has also been equipped with more powerful servo motors and hydraulic systems, making it far faster and stronger than the T-800 Series. It can therefore sustain far greater damage and continue to function, and is more deadly in hand to hand combat, even being able to hold its own for a time against the T-X Series in pure physical combat."
Jesus christ.