Is 级 necessary in 优先级
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.