...One other thing that does concern me a bit is the loss of focal length going to a full frame - i have read that my SAL70400G on the A77 would be like a 600 - that being said does anyone use teleconverters, how easy are they to use. ...
The effect of using a lens on a body with an APS-C sized sensor vs a FF sensor is one of the worst things for misinformation on the internet!
The Focal length of a lens DOES NOT CHANGE when you put the lens on a different body, it is an optical property of the lens.
What does change is the Field of View - IE how much of the scene is visible.
The smaller sensor only 'sees' part of the image, IE it 'crops' an image from the full image (hence APS-C cameras often being called 'Crop' bodies).
The reduction in Field of View is such that you get a Field of View equivalent to that for a lens with a Focal Length 1.5 times that of the lens - So on a 400mm lens you get the field of view as if it were a 600mm lens.
There is an apparent
increase in magnification as if the focal length were increased as the viewfinder shows the image applicable to the sensor size - but all you are seeing is a crop (though this may be useful for aiding focus on distant objects).
There is also an apparent
increase in magnification on viewing the resultant image - and this is where direct comparisons become more complex, as it depends on the specifications of the two sensors.
If the FF & APS-C sensors have the same pixel size (EG A900 & A700), then there will be no difference between an image shot on the APS-C camera, a shot from the FF camera in crop mode, or a post processing crop of an image from the FF camera to match the APS-C resolution.
However, APS-C cameras often have smaller pixels than FF cameras, IE an APS-C crop from a FF will be lower resolution than the shot from the FF-Camera.
This gives an apparent
magnification for the APS-C camera BUT
you get a similar apparent
magnification comparing a FF shot on an A99 (24MP FF) to that on an A99II (42MP FF) - if you crop an A99II image to 24MP you get a more 'magnified' image than the 24MP image on an A99 with the same lens.
The A99II in APS-C mode is ~17.8MP, so you would 'lose' a little compared to the A77, but nothing like the 1.5 x that the simple 'FF vs Crop' conversion appears, and the benefits the A99II gives would easily outweigh this, I suspect!