What is the manifold pressure with and without lean cruise enabled. If the vacuum increases (kpa value goes down) then your engine is running more efficiently. Once you work out what the engine wants, then experiment with timing in those areas too. Lean mixtures burn slower and need more spark advance, again keep an eye on manifold pressure. It's the best way to work out if your motor is happy. This is what I found when I tuned my Megasquirted Pontiac 400 engine and I applied the same logic to my 2005 Holden Monaro CV8Z LS1 engine and seems to work great.

In my LS1, I cruise around 15.5-16.0 AFR with 38* spark lead. But the 400 can only take ~14.8 due to cam overlap.