To keep this short (considering this has been discussed in length before...)
Yes, it would be possible to implement function calls to work with GET requests. You'd have to break the REST principles for it thought, which I for one can't seem to find a valid enough reason for just yet.
It's not so much about the size as it is about what you intend to do with it.
If 'speeding up development' depends on abusing certain conventions, then maybe you're going at it the wrong way. The fact that a browser just so happens to use the same 'thing' (GET) for it to work is a happy coincidence, but does not make a browser a development tool as such.
There are various other routes you can take to call these functions without depending on breaking REST. There's the CLI (which is a good thing to install regardless), Particle Dev, and countless webpages made by community members that will allow you to quickly and easily request variables and trigger functions as needed. I've linked you to mine before, but here's one by @suda. Heck, some people even made apps for this specifically.
Any other reasons why we'd definitely need GET to work as POST that we might have overlooked in this, and previous, discussions?