Is 级 necessary in 优先级 From the dictionary I gather that 优先 by itself means "priority" and so 优先级 means "level of priority".

In a phrase such as 高优先级，or 最高优先级，is the 级 necessary to sound correct, or is it an optional "stylistic" element, as in English where "highest priority" is the same as "highest level of priority"?
Unlike English, （简）优先／（繁）優先 is an adjective while （简）优先级／（繁）優先級 is a noun.

1. （简）这个项目优先级最高。
   （繁）這個項目優先級最高。


If you want to use an adjective then the expression will be like

2. （简）这个项目是最高优先的。（所以……）
   （繁）這個項目是最高優先的。（所以……）


You may notice a small difference in meaning, the first one is often used as response or a single sentence. The second is NOT very usually used as only a single sentence, but you can still use it as a single when the 所以 is obvious.