(1) They reduce wing bending moments due to moving mass out from the centre of the aircraft. Consequently they should extend the fatigue life of the upper wing skins, mainspar and D-box.
(2) They tend to prevent swirl around the wingtip, which reduces induced drag.
(3) They tend to increase profile drag, primarily through interference drag at the attachment point. This however is generally less than the reduction in induced drag.
(4) ESDU data sheets are the best place for such wisdom, the abstract for the sheet you need is at
http://www.esdu.com/cgi-bin/ps.pl?se...t&t=di&p=97017 although I'm afraid that you'll need to either buy the sheet you need (they're a mercenary bunch at ESDU) or get it through a good technical library. If you do a search on the ESDU website you'll probably find some other relevant data sheets.
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