BIG genoas are VERY inefficient for beating and require a LOT of 'slot' distance to optimize the airstream flow over the main. How to tell that the genoa is too big .... watch the leech tell tales and if the lee side leech tales are drooping = the flow at the leech is now 'separating' from the genoa at the leech. The main then cannot 'bootstrap' (velocity dumping) with the genoa properly ... and the whole sail combo 'fizzles'. Try using a smaller Genoa when you are racing, especially on windward/leeward courses.
You can 'peel' to a large genoa 'after' the windward mark if this is a 'no spinnaker' race.