Your friend works/worked for Honsowetz/Pink/Taguchi/Hermann? What is your friend's name?
An engine delivers X power (repeatable) @ Y width @ Z longevity. GTP wins are a combination of gearing, aero, chassis and suspension setups, component suppliers (say tires can't deal with power peaks, so lose the peaks and pick up on width) people interpreting the car subjectively and objectively, driver.. and a million other things. Difficult to pick a single factor out from a whole group of them and make a comparison and make a conclusion.
I think the Japanese have done a wonderful job with their rally car powerplants. I don't see them as inferior to German stuff. I think German cars have built a reputation because they used to really overbuild their stuff. This trend is going away though. Look at how the VAG 1.8T has lost weight and component strength over the years, how BMW has moved away from cast iron inline 6s and stock I4s that ran 1400+hp in F1, etc. Most Japanese cars are built light, so their engines usually are too, but when they decide they want to make an engine that takes it all, they come up with stuff like the RB26DETT, 2JZ-GTE, 4G63, etc... Anvils!
What problems do Toyota heads have?
The SG race industry deals mainly with Japanese stuff. Local companies may not be able to spec and build as good as a company in Germany that builds entire crate engines for big money and ships them to Singapore, can. So again it is difficult to say that the Japanese stuff has serious issues. If there were a local German tuner and builder that operated at budgets similar to local Japanese tuners, then you might have a more accurate comparison.
Cheers
