aproposof This comes down to the limitations on using certain verb forms to refer back to earlier verbs.
*If we use HAVE BEEN, it needs to refer back to another form of TO BE.
Correct: "I
am happier now than I
have been for months."
Incorrect: "I
feel happier now than I
have been for months."
*If we just use HAVE to form present perfect, it can refer back to almost any previous verb:
"I plan to
eat more tonight than I
have for the last few weeks."
"Our donors expect to
award more grants this year than they
have in the past."
Notice, however, that when we do this, it's important to have a modifier on the second term to clarify the comparison. If I just say "Speak more kindly than you have," it isn't really clear. Am I saying to speak more kindly than you EVER have in your life, or than ever have to me in particular, or than you have in the past 15 minutes? The "in the past" above might seem vague, but it does convey the idea that we are comparing to what has been typical throughout the past, rather than just a particular recent stretch of time. The modifier also prevents a logic error: technically, if you say "I plan to eat more than I have ever eaten," you need to exclude that present time from the comparison. This is the same logic that prevents us from saying "China has a larger population than any country." Without OTHER, we're saying that China also has a larger population than itself! I can only leave out that modifier if I am comparing two different entities: "Elena has learned more physics than I have." But even then, if the first part is not in the present tense, we need a clarifying modifier: "Last year, Elena learned more physics than I EVER have."
*If we use HAVE + VERB to form present perfect, it can still refer back to almost any previous verb, but if the verb needs other elements to make sense (e.g. an object or modifier), then we need those elements, too. This is partly because if we didn't want to specify those parts, we would leave out the second verb.
*"I plan to eat more tonight than I have eaten before." (Since there is no object or modifier for "eaten" other than the timeframe, that's all we need.)
*"Valentin has sent more letters to the president than I have sent to my parents." (If I want to compare how many letters *I* have sent to the president, then I should say "than I have" and leave it at that. I could technically say "than I have sent to the president," but it's quite repetitive. The only reason to say "have sent" is to adjust the object or modifier.)
By these standards, both of the forms you propose have flaws:
*"to set dividends more conservatively than they have"
-This is like our second case (HAVE w/o verb). Some folks have proposed that we need the rest of the phrase: "than they have been setting them." That might work if we include the whole thing, but as we saw above, we could also use a simple modifier "than they have in the past." Regardless, the bare "have" won't work.
*"to set dividends more conservatively than they have set""
-This is like our third case (HAVE + verb). We need to either put in the rest ("than they have set them in the past"--redundant but grammatical) or ditch the verb "set" and add a clarifying modifier ("than they have before"). As it stands, this is not proper English.