Some but not all. If the tweak or app directly mods the core files, it won't show up. But in the case of apps/tweaks, yes they'll show up as others in usage. If you want a more accurate projection, you can use Cydia's breakdown to give you a more detailed breakdown of it (really nice feature to be honest). But there shouldn't be that much of a change in your cores since they're very light changes. I think the highest I've ever peaked on my core changes was about 1.16 GB of the system storage, and that was because I used both Cydia and personal tweaks, so it racks up a lot faster. But normally, you probably won't get above a 1.05 usage of your system file even if you're heavily tweaked/modded