optimize pg_tables query ( text vs varchar ) ...why ?

Hi
For DDL purposes we make significant use of pg_catalog tables/views.
Were investigating performance issues in a typical function:

CREATE FUNCTION tableexists( s TEXT , t TEXT ) returns boolean as
$$
SELECT count(tablename) = 1 FROM pg_tables WHERE schemaname=$1 and
tablename=$2;
$$
language sql

When change the params of above function to VARCHAR (instead of TEXT),
performance improved dramatically.
We then changed params to NAME ( as per pg_tables column type ) , but the
performance stayed more or less the same.

Can somebody explain this to me ? Is there a better way in which to handle
these ?
(This will be implemented on most object in the catalog e.g. columns,
sequences, functions, etc )

Regards
gmb

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