The texture is of course only saved as one copy in the video-memory,
for all instances of a model. (anything else would be foolish engine programming)
So you have to make clones of the houses, where each model
is a unique sperated modelfile. (just make copies of the modelfiles, and add a numbers to them)

To make a reliable performance test you should create as many dummy models,
as you will later use in the final game. The dummy models should have the same
polycount and texturesize as the planned models.

Since you probably also make LOD versions of the models
(else the town would be terribly slow),
you have also to account for the (smaller) LOD versions of each house.

Also: make dummycalculations to
account for the AI etc. affecting the CPU usage. If every person uses a collisionmovement, you
should do so with dummy models, that walk around randomly.

For the performance test, you should test a running demo
on a variety of computers, to see the Framerate.