> Given that the e5500 has improved fpu performance

As far as I understand, the e5500 has improved FPU performance towards the e500mc, where the FPU is crippled by running only half-clocked. Thus, the e5500's full-clocked FPU would be no better than what's considered a usual FPU elsewhere (including other PPCs with real FPU).

> what is the impact of the missing Altivec instructions
> (in particular in regard to MorphOS).

That question is best answered by a MorphOS Team member. But to get an idea about usage of AltiVec by MorphOS components:
http://www.google.com/search?q=site:morphos-team.net+altivec
Of course, there is also 3rd party software leveraging AltiVec if available, like MPlayer or dnetc client. But I think there's hardly any MorphOS software that really demands an AltiVec unit to be present. It's just that in case of missing AltiVec generic code is used und therefore the program is running slower.

> as a 64 bit processor, could MorphOS be ported to
> the e5500

I think it could. And it could even use the 32 bit mode of the e5500. So no need to port MorphOS to 64 bit.

> and how would performance compare to a G5?

Good question. According to Freescale the e5500 delivers 3.0 DMIPS/MHz, for the PPC970 I found 2.9 DMIPS/MHz (presumably ignoring AltiVec).