Stop me if I'm wrong, the Desire(Nexus One) is a first gen Snapdragon, with an Adreno 200. The Desire HD is a second gen Snapdragon with an Adreno 205 GPU. The 205 is around 2x as fast as the 200. So technically, the Desire HD is substantially more 'capable'. Not sure how much RAM each device has.
Is Sense fully hardware accelerated? I know this whole GPU argument that people use, but given that Android is not very hardware accelerated, most of the differences is really in games/3D applications.
Your day to day use such as opening emails, surfing the web, etc, are almost all handled by the CPU. And sure the newer devices score higher Quadrant scores, but I suspect the boost comes from a better GPU score AND optimizations. We've all seen Nexus Ones in the 1500+ Quadrant scores too.
I could be wrong though. The 205 is definitely more powerful, but I dont know how much this Sense stuff is attributed to the GPU. I'm not saying just from a hardware perspective, but HTC has consistently churned out newer Sense versions with newer phones... Basically every iteration gets a newer version and every older one gets left in the dust... of course ROM cookers show that its still possible for the old phones to run fine on a newer Sense version.
It's understandable that manufacturers do this and abandon older phones, but it'd be nice if they could have solid sense builds that get ported cross platforms. HTC releases new phones every few months, and so it's kinda annoying when you get a new version in 3 months. The Incredible S gets a nice Sense upgrade, but it's not that much different from a Desire HD for example.