Making sidewalks as .mdl's

The reason I want to build sidewalks, and city blocks in Max is to have more control over how the sidewalks and curbs look. Will this decrease performance in game?

Models are cheaper to render than the equivalent brush geometry in most cases, so no, it shouldn't hurt performance (though model lighting can sometimes eat up a lot of resources, so beware!). Compare a level with models and brushes if you're still worried.