Our experience with 30' was a few top pitchers were dominating, the decent ones could throw strikes without being in danger and the weaker ones struggled to throw strikes (which resulted in coach pitch instead of BB).I think I liked 35 and it worked OK. I think I would be worried about the pitcher getting hurt by a ball hit back at her a 30' more so then her pitching but I have never done it.
Since 30' is the standard here, all the leagues have pitching plates at that distance. It seems other areas of the country just starting kid pitch at 8U tend to just use their existing 35' plates.