The carbonate solution won't make your developer higher contrast. It will make it more active, which means it will be a bit faster developing your photos. But, keep in mind, the more of a 5% carbonate solution you add to your developer, the more you dilute your developing agents.
Adding carbonate + some restrainer (benzotriazole or potassium bromide) can boost contrast, since the boosted activity will ensure that more dense parts of the print develop fairly quickly while the restrainer holds back the less dense parts. Benzo in particular does not restrain "evenly" -- it really slows down the development of the less dense parts (of a print or a negative). Too much kills the developer, though.
What a 5% solution of carbonate does accomplish on its own, however, is boosting your developer when it should probably be poured down the drain. But only a little and only for a short time. That may just result in more-quickly developing muddy prints, though.