wow, surprised no1 has said this yet.

your miss rate increases with the level of the mob, up to raid bosses (which are classed as level 63)

to get a 100%hit rate you need 8.66%hit in total.  any more than this is pointless and uneeded.  This value is easier to round up to 9%.

weapon skill gives 0.25%hit (and 0.2%crit) per point over 300 I think, if any1 is interested (not 100% sure on this though)

now, as im sure alot of you are aware we shoot tranq shot on a few bosses, missing = bad news, so you should aim for 9% hit so that you are reliable as possible(tranq can still fail)

this scope is better than a +7 scope if you are lack 2 or more +hit (i.e. if you have 7 or less)

also, dont forget, slower weapons offer better dps, BUT +7scope on slower weapons offers a smaller dps increase.

so you chooce, yes it is expensive but also very useful if used correctly.