Lou, you have actually picked what I consider the best dates to goto Ibiza. Ive been in the middle of Aug, end of Sept/Oct (closing parties) and mid June into July.
August was FAR to busy! Queues to get in clubs wrapping round the building, bars rammed to the doors meaning ages to get served and dance floors so packed you literally cant move your arms from your side. Finally the temp is blazing hot, you know that heat where you just cant stay out longer than an hour in the sun because you think you might die and no matter how much sun cream and **** you put on you always burn....not nice.
Sept/Oct was the complete opposite, pretty much every night was dead with clubs half full, some almost empty. The closing parties are great but I prefer the openers myself. Was quite overcast and rained a bit when we went then to.
Late June to early July Ive been four times now and you catch some of the opening parties which are busy but nothing like August. Everything is open and as its start of the season I think more of the workers are on a high and happy

Oh and ROOM TO DANCE IN THE FRICKING CLUBS! Ah and the sun is hot enough to get a nice tan without melting your face off.
Actually am doing similar dates this year! see ya there
tom