Look up about rigging your sail properly online, theres lots of advice.
Slalom and race kit can be very specific to what mast you need and what board it will suit best too and its usually not the quickest to get you going either as the proper race guys always sail completely maxed out so although they are using a 7m sail it is in winds that us mere mortals would be on a 5.2m sail.
Get yourself a caM-less free ride.
Also get a matching mast, this can make a huge difference to how the sail performs. Use the link below as some sails just don't work on there masts.
www.unifiber.net/masts-selectorYou will also highly likely need a longer mast. Do not get a mast that you need a long extension for IE: don't use a mast that needs like a 45cm extension. I always only use an extension 30cm Max on any mast.
The sail shape you described was right, you want shape where you hold it and not higher up in fact you want to back edge to be a bit flappy, its called exhausting the wind. If theres too much wind to handle in a gust the sail breathes it out a little. You also want the battens to rotate around the mast on the bottom two, these will then lift away as soon as the wind fills it, like halfway around the mast.
Sometimes sail measurements can be a little out too especially if you are using a different mast.
Usually too the board will have a sail range and its usually written on the board, this is usually for an average person of 85kg and this is usually the limit that the board works at so if it says "4.5m - 7m" don't think it will be any good with an 8m sail on it.
Good luck with finding some gear. If you get stuck or see something you might want to buy then Message me details.