I want to have 2.7K footage stabilised in Full HD
Well you won't loose pixels if you nest it.  Nest it. Then change the sequence settings of your nest to match your clip settings. Then run warp on the clip in the nest. 

Back in your main comp you can set the sequence to full HD and just scale it down. You will get the extra sharpness of 2.7k as it will be in the HD timeline.

If you have a majority of the clips in 2.7k then it might be best to edit in 2.7k and drag that composition into a new composition with full HD settings and scale that instead of doing it on a clip by clip basis. Which can get tedious. 

Or you can just edit in 2.7k and export in HD.

Side note: you always loose a little bit of pixels if you stabilize (this can be avoided to a certain extent if you adjust the settings albeit at the detriment to the actual stabilization). But downscaling to 1080p it won't be noticeable. Especially with gopro footage. 

I've also have mercalli does better with some clips than warp. It's available as a plug in for premiere.