If its for occasional use the best option is the Sea Sucker Talon imho
http://www.seasucker.com/shop/1187/
When a roof rack is not in use, it causes unnecessary drag and wind resistance which ultimately effects your fuel ecconony as such if one of the requirements is occasional use, removing it is ideal.
I have just sold a set of Thule 591as IMHO the sea suckers hold the bike closer to the car which results in the bike being buffeted about less and the fact you can move them between cars in seconds is more convenient than you would first think.
I have done similar speeds with them to what I did with the 591 without issue I'm very impressed with the product.