In the documentation for LPAD and RPAD is written that the result type is VARCHAR(32765) and there is a tip to cast the result to a useful length.

When using LPAD or RPAD in a "SELECT LPAD( LABEL, 8, '-' ) FROM...", FlameRobin and IBExpert both return a VARCHAR(8), corresponding to the length argument of xPAD(), without casting.

My question: is the cast still recommended or obsolete?

I use the internal functions. Is the behavior different if I use the external functions from ib_udf.dll?

Server Version: Firebird 2.1.4.18368

Reference: "Firebird 2.1 Language Reference Update" from 2010-12-09, document version 1.0 — covers Firebird 2.1–2.1.4
http://www.firebirdsql.org/refdocs/langrefupd21-intfunc-lpad.html