The 7-14mm is the reason why I went with the E-system. You get the equivalent of 14mm in terms of 35mm format.
Anyone that has ever used the Nikon 15mm or the even better 15mm Zeiss Distagon(IMHO it is the best ultra wide ever made), you would realize that Olympus 7-14 is quite reasonably priced for it performance.
Adapters are well... adpaters and will never perform on par with a lens, even when compared to a Sigma.
All I can say from my expensive experiences, Ouch!, when it comes to wide angles lens, buy it cheap but it twice. Many times in the past, I should have just saved up a little longer and go for the best wide angle lens.