That's where you get close to straddling that thin line, ya know!?
I would try 400 but when you start to get much above that, you could get close to the point where the Alum. begins to soften? I believe 600-650 is where you can go from case to blob...so 400 or even 450 (? ) should be OK...but I wouldn't guarantee it.

I guess one way to look at it though (at least the way I would look at it if it were my engine ) is if you can't get it out, the case is no good anyway...so go for it.

Don't know if the guys at Horizon would know either?

There is a "Club Saito" in the glow engines forum...there probably would be some more suggestions if you ask there.