If you are looking for a speed improvement with wheel pants, forget it.
Not quite true, with the 100HP 150 the gain may not have been significant, but notable. Definitely notable with the 150/150. Pants will also cut your useful load.
By the book, yes, but if 18 pounds is going to make or break having a safe flight, it would be best just to stay on the ground anyways. 
$150 also buys a bunch of fuel.
One cross country flight? 
Otherwise $150 for the pants, $100+ for mounting brackets and probably $100+ for shipping, ($200 to the west coast. 
) Wheel pants also run up hours spent during an annual.
Remove them before taking your airplane in? On an owner assisted annual it wouldn't make an difference. 
