Almost 40 years ago, Arvel Gentry wrote an article for SAIL that still remains the definitive take on staysails...
go to this link, and open the PDF file entitled "The Double Head Rig":
Staysails can be awesome...
ABSOLUTELY "the" most definitive article ever written regarding 'staysails', especially when flown 'under' a topsail or headsail, etc. Here's the direct link: http://arvelgentry.com/magaz/The_Double_Head_Rig.pdf
Caution: this article is referenced to 'true' cutter rigs with the mast at ~50% LOA and the with combined "CE" within the staysail or 'in front' of the mast.
The disadvantage of a staysail is realized in 'light' conditions when beating .... usually too much 'interference' between topsail and staysl ... @ less than ~6kts. on most boats.
As the article implies, the cord length SHAPE of the staysl (draft forward) is important ... not something that is easily attained from a sail loft that designed the staysail in similar geometry for a 'sloop' headsail/jib; I like my staysails to be draft-forward and with FLAT luff entry.
The 'mechanical / rigging' problem with flying staysails is the 'interplay' of headstay/forestay tensions ... in reactance to a single backstay. For the purposes of this/my discussion - the topsail is flown on a HEADstay - the furthest forward stay; the staysail (FORESTAYsail or staysl) flies from the FOREstay - the stay that is 'immediately forward' of the mast.
This problem usually results in the headstay loosening with increasing forestay tension / loading ... and can cause significant luff sag on the 'topsail' / headsail.
The pragmatic solution is to fly the staysail on a quite loose ~5-8% tensioned forestay, which automatically tightens the headstay .... only using full forestay tension when the headsail is doused or not flown. If you dont 'consider' the resultant sag to leeward of the headstay when 'reacting with the forestay', the cutter rig will point poorly, heel aggressively and can easily skid to leeward on an artificially 'powered-up' topsail/headsail. Once you are cognizant of the two forward stays have a 'tension interplay' problem ... you can make a cutter rig 'point' like a banshee --- mostly by loosening that FOREstay.
To make a staysail / headsail combo 'really work'... you really need running backstays to 'help' with the 'two forward shroud' tension interplay problem.