Well, I figure a single RAC/20 could work on a King Crab-like urban assault unit and, of course, on an ASF. But in general I agree, it's more of cool piece of fluff than a practical weapon. A RAC/10 is much more practical.
A LAC/20 is rather iffy due to the range issue I talked about. Once you get below 10 hexes of range the fairly linear correlation between range and general efficiency doesn't hold any longer, 2/4/6 range isn't 2/3rds as effective as 3/6/9. To be balanced for specialized units the LAC/20 would have to be almost as heavy as the standard AC/20. At a quick guess I'd say something like 12 tons, 5 heat.