ECM Tuning for MACK trucks

We have started tuning ECM's in Mack trucks and have been somewhat surprised at the improvements possible in fuel savings and/or power increases. There does not seem to be a huge amount of interest and I am wondering if this is

a) because Macks are already great of fuel and power or

b) do I have a lot of competition?

I rarely do cars now days as the competition has taken prices beow my costs so I am wondering if this is the case for Macks