Hey Peter,
For starters, leave your 15m at home. We just don't use them here ..... ever!
Bring the 10m and 13m. You may use the 13m from time to time but we get very little increase in wind speed in the afternoons, it's just not blowing one minute, and cranking the next. We generally get 18 to 23 knots most days and often up to as much as high twenties to thirty knots.
It does increase very slowly throughout the afternoon, but only slightly maybe by 5 knots all afternoon from the initial onslaught. Gotta love WA wind. If you need any more wind, just head north!
Scarborough is your best bet for restaurants, bars and close to the Brighton Beach surf spot. There is lots of accomodation around Brighton/Scarborough.
Enjoy Perth, it's damn windy in Dec/Jan.

You should ride at least 6 days out of 7.
Cheers,
KH