Can GiST for tsvector be optimised? #TextSearch

Hello Dear People,
I have a few questions about the use of GiST for text search.
My situation is this:

I have a large and rapidly growing database of documents, and I want to support text search.

The documents contain lots of words that it would be valid to search over. I think this means a GIN index will become too large, and a lossy index is
better.

I've tried setting up a GiST search (just using the default English configuration). The performance is disappointing.

The GiST/text-search documentation says "GiST indexes are lossy because each document is represented in the index by a fixed-length signature."

Is there a way to configure the signature which GiST + tsvector is using?
Presumably a longer signature would produce a larger but faster index?
Where should I look to find more information on what exactly GiST is doing for the text search case?

Thank you in advance for all help!
Best regards,
- Daniel

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Dr Daniel Winterstein
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http://winterwell.com http://sodash.com
Registered in Scotland, company no. SC342991