Just wanted to add: in whole your discussion you are treating video memory as "infinite". But what will happen when you try to create too many textures in POOL_DEFAULT? Hint: it will fail, but MANAGED textures will be created and D3D runtime will guarantire what managed textures will always be usable by applicaiton and in any time.

I mean if you want to manage textures by yourself - it's fine. But you should be aware about much more stuff in exhange of smallter application memory footprint. But if you limited by system memory - most probably all your system copies of manages textures will be flushed to swap file after some time - so they will not reduce perfromance in any way.