While it is true that simple modeling like this is adding unseen faces and such, it is unlikely that it will affect performance as much as I think they're making it sound. Gamestudio uses an effective culling algorithm that does not render unseen geometry.

Instead, your performance issues are probably related more to the fact that the simple modeling is using more video memory. More shapes take up more space. If your intended user is like to have a high-level system, this shouldn't be a problem.

There is a trade-off here: simple shapes tend to be easier to texture, both for you (the creator) and the computer (the renderer).

My recommendation is to try making a few "complex" models and compare them to your simple models. There's a couple of things you'd probably want to consider:
1) Am I getting a noticeable performance increase?
2) How long is it taking me to make these new models? Is the extra time worth the gains?
3) Do the models look good? Are they matching my intended style? Are the textures looking okay?

See how things go. Looks good otherwise!

(Edit) Sorry, I saw something else I wanted to address. As long as you are using the same textures repeatedly, there shouldn't be a performance decrease. The texture images are loaded in the video memory once, then used over and over again on the models. Again, this might be another reason that simple modeling is good, since you do not need an individual texture for each model.