That works, but it's not quite the same.
Unless a comma (or dash) is present somewhere, or unless it ends a sentence, I almost always see a "than" follow a "less" in a sentence. [link|http://www2.ncsu.edu/ncsu/grammar/Pronoun3.html|E.g.]:
No man is an island, entire of itself; every man is a piece of the continent, a part of the main. If a clod be washed away by the sea, Europe is the less, as well as if a promontory were, as well as if a manor of thy friends or of thine own were. Any man's death diminishes me, because I am involved in mankind, and therefore never send to know for whom the bell tolls; it tolls for thee.
I find 79,000 hits for "as less as" (in quotes) on Google. In the examples I see, I would be most likely to use "as little as" (37.7 M hits) or "as few as" (4.9 M hits) instead.
Skipping over "only slightly less" dramatically changes the meaning for me. Instead of it being a comparison, it lumps them together. "I regard X as I regard Y." If I couldn't use "than", I would have expressed that as, something like, "I have less regard for X; like I have little regard for Y." In other words, I think the "less" is being used as an implied comparison, not to indicate near equality of regard.
Short subject lines are a bitch.
I'm not saying Peter's wrong. Just that it is an expression I don't recall seeing before. English is neat like that. :-)
Cheers,
Scott.