Betaine may be used in improving the sequencing reaction of GC-rich templates. But, what is important, it may "introduce" false heterozygotes and there are more "N" in the chromatogram, so it may not very useful if you are interested in mutation identification. Then, other additives are more useful.

I know betaine for GC rich templates and long PCRs, not actually specificity. For the latter issue TMAC (tetramethylammonium chloride) is a quite useful additive.

Also have a look on the concentrations as this has also an influences the effect of an additive.

The additives I use most often for purposes of increasing specificity include DMSO or formamide.