There's still a lot of misunderstanding out there about where the Halo Wars engine technology came from. Starting in very early 2005 the HW team wrote a new engine pretty much from scratch. The Age3 code was only single threaded, didn't use SIMD, and consumed huge amounts of RAM. (Age3 used over 32MB just for UTF16 strings - not good for a console game!) The "Bang!" engine ran at ~7Hz and took around three to five minutes to load on the early Xbox 360 devkits.
Colt McAnlis (now Google), Billy Khan (now at Id Software) and I wrote the entire Xbox 360-only renderer almost from scratch. We started out with Age3's particle renderer and my "wrench" demo deferred shading engine for SM 2.0 hardware. Ensemble Studios basically gave us a blank check to do whatever we wanted on Xbox 360. (What good times!)
Age3's particle engine (written partially or mostly by Graham Devine, now at Magic Leap) was so good that the artists refused to allow us to rewrite it. Billy and I threaded it by converting it into jobs, and we SIMD'ified all the key loops using Altivec ops. We also offloaded as many computations as we could into vertex/pixel shaders, to cut down on the very high CPU cost of the original code.
The Halo Wars particle engine would have ran circles around Age3's (once ported back to x86).
Please don't get me wrong, Age3 was a beautiful and fun game, and I loved working on it. The team was super easy and pleasant to work with. Just remember that Halo Wars was created by a very different team with different goals. We had some pretty awesome goals for the next Halo Wars, but the studio was shut down.