Higher mash efficiency will give you a higher gravity and eventually higher abv all things considered, and can alter the IBU and perceived bitterness of the finished beer.
You really want to shoot for the efficiency that the recipe is built on. If the recipe is designed for 75% efficiency and you're hitting 80ish you're still in the ballpark. Much higher and you may want to consider adjusting the hops or bringing your efficiency down by either reducing the grist bill or adjusting out your mill gap.
Some including @dmtaylo2 believe adjusting the grain bill too much to compensate for high efficiency will leave you with a more watery tasting finished beer. Some brewers like to brew that way because they feel they save $$ in the long run by reducing their grain bills. I've been creating recipes based on 80% efficiency and opening my mill gap a little at a time to attempt to bring my efficiency closer to that range.
The key in my opinion is dialing in the recipe to your efficiency and finding consistency to produce the best beer. High efficiency just for the sake of saying..."my efficiency is 90+ percent" is really nothing to get caught up on.
Remember I'm talking about MASH efficiency here. Brewhouse efficiency is another thing entirely and is essentially based on beginning and ending volumes.