I'm thinking about buying a new reel for my weight 8 rod (which I use for steelhead), but want also to use it for my weight 5 rod (which I use for trout). The reel is supposed to be used with lines weight 7 to 9, but I wonder if its performance will be affected if I use a lighter line. Anyone there who knows if it would work?
I assume you are considering a new reel with multiple spools. The reel's performance will not be affected except that with a lighter line you will be able to fit more backing. In a 5wt setup that usually wouldn't offer any benefit.
One potential issue with using a much larger reel is the balance. You may be able to cast well with a poorly balanced setup, but it will get very tiresome over a long day.