Am an expert? Not sure, but I worked on an enterprise search spider and search engine for about a decade (Ultraseek Server) and Ive done customer-facing search for another 6+ years.
Let the server reject URLs it cannot handle. Great servers will return a 414, good servers will return a 400, broken servers will return a 500, and crapulous servers will hang. In nearly all cases, youll get a fast fail which wont hurt other users of the site.
Manage your site for zero errors, so you can fix the queries that are too long.
At Chegg, we have people paste entire homework problems into the search for homework solutions, and, yes, we have a few queries longer than 8K. But we deal with it gracefully.
Never do POST for a read-only request. Never. That only guarantees that you cannot reproduce the problem by looking at the logs.
If your design requires extremely long GET requests, you may need to re-think your design.
Walter (my blog)