max RPM while compression braking

[Idahotom]

I live on a mountain with a long 12% downgrade. Coming down with my 5 sp. '08 HB, I of course am in DFCO the entire way, that's the good part.

I am unsure what the redline is of the Yaris engine, keeping in mind that running at redline while going downhill it would seem be easier on the engine then at the same RPM while pulling hard. I hate to ride my brakes, or shift down to avoid riding the brakes and then be going 25 mph (people drive the hill at 40 to 55).

What I'd really like is a compression brake like I have on my truck, but that ain't gonna happen! So... any thoughts on the max permissable rpm while in downhill/FDCO mode? I'll brake if I have to, brakes are cheaper then a new engine.
Tom