I would say that a possible 25 horsepower is not a great deal when you consider that you are turning up the rpm from 5350 to 5650 and increasing timing advance on the motor to achieve it. As for the retarded timing at low rpms you and I both know Mercury's fondness for that however thats not going to generate any real high midrange or top end power for these engines at rpms where the timing curves are actually retarding timing as they should from midrange advanced timing.
When you consider that Mercury Racing has had these engines in the field since 2001 with a lot of them in all out racing applications it becomes more important to maintain a little conservatisim in the programming to deliver good reasonable engine life of 300-500 hours. As we all know the HP series engines have always had a mild problem with valve spring life and this is aggravated when the HP525 is going to be run at 300 rpms more on the top end.
Almost any engine can be pushed to the ragged edge for maximum power however most of the time the engine owner will loose valuable reliability and engine life. If they are racing thats a whole other situation and sometimes winning means running at the EDGE!
I feel that if you have an HP525 series engine and you want a significant horsepower increase of lets say 50 to 100HP more then you should be prepared to go into the engine and do some cam, headwork and even consider taking the engine from 502 cubic inches to a 540 cubic inch version and then do the necessary upgrades to the fuel injection system to support the added power. These type of changes will for sure require a reprogram of the ECM to allow a 575HP to 625HP version of this engine to operate reliably at its original 5300-5400 rpm range.
I personaly don't feel any big block chevrolet marine high performance engine should be operated at sustained rpms above 5400 rpms for long periods of operational time without a shaft rocker system and special valve spring packages and valves to keep the valve train stable with this type of endurance usuage. You can do it for awhile, but eventually you are going to pay with a badly damaged engine!
Every performance boater is thrilled with his new found performance until that little bugger grenades!!
Just my two cents and maybe not worth much more.
Ray @ Raylar