Reduction SAT to a problem on a planar graph with as few vertices as possible
You are right that improved reductions from CNF-SAT to any one of various planar graph problems would give improved algorithms for CNF-SAT (via graph algorithms with runtimes exponential in treewidth; such algorithms exist for many graph problems). If you could get $|V(G)| = o(m^2)$ in the reduction you mention, this would imply that the Exponential Time Hypothesis (ETH) is false. This is well-appreciated, yet it has not led to any improvements on running times for CNF-SAT (as far as I know).  For more on how ETH has been used to classify the difficulty of planar (and non-planar) graph problems, see e.g. this survey of Lokshtanov, Marx, and Saurabh.

As for reducing the constant in such reductions, achieving $|V(G)| = c m^2$ for $c$ as small as possible, I don't know how much effort has been devoted to this.