When I got my Strat, I had a new nut put in that radiused the strings at 12" and perhaps raised them slightly (maybe 1/16") I also had the string spacing set as wide as possible. The bridge was also radiused at 12". However, I don't think I'd like a concave
slide very much.
I tend to think of a slide much in the way a fiddler thinks of his bow. It's a rare thing you're playing ALL the strings at the same time and the straight slide allows you to "bow" the strings, even though it's with your left hand, something like a fiddler bows the fiddle strings.
Of course everybody's different and if concave floats your boat, by all means use it, but this is another way of approaching it.