Grassroots motorsports did a wheels weight comparison test in their 6/2003 issue (should be on shelves now). They used a Mazad Miata and g-Force T/A KDs for their test. They only compared simlar size wheels...did not plus size. Here are the results:
Kosei K1 Racing 15x7 wheels - 13.5 lbs.
Lap Time (AutoX Course): 39.245 seconds.
MSW Type 33 15x7 wheels - 22 lbs.
Lap Time (AutoX Course): 39.428 seconds.
They mentioned that road racing will result in even larger loses in times.
Grassroots Motorsports also has an article on their website that compares plus sizing. Good info...http://www.grassrootsmotorsports.com/plustest.html