I will say this hobby is a lot more fun when one doesn't find out about all these aberrations and just enjoys the views 
Part of learning to see is learning to see aberrations... It's my choice whether or not they bother me.
Not long ago, I was looking through my stuff and I found a 30 mm GSO SuperView I hadn't seen for quite a number of years. It's a basic eyepiece with no field stop and a lot of off-axis astigmatism. But I have been using it and enjoying it.
The 15mm 66 degree at F/4.5... I have had several of these eyepieces. They came out more than 20 years ago, right after the Orion Expanse series. They were the first affordable wide fields. I bought the 6mm, 9mm and 15 mm as Adorama had a buy 2 and get one free deal..
My main scope at the time was my 12.5 inch F/4.06. With a Paracorr, it is F/4.67. The 15mm showed a lot of off-axis astigmatism as did the 20mm. The 6mm and the 9mm are negative-positive eyepieces, a negative doublet in the barrel, they are much better in a fast scope. In an 80mm F/11.3 refractor, the 15mm and 20mm are quite good.
Jon