The multi-poster can send an unlimited amount of topics, not "250". Im pretty sure the thema poster works the same. That limit that you are talking about is as a result of the maximum amount of threads an application can use "per core" - so technically a quad-core can utilize 255x4 threads - however, the MP7 uses asynchronous threading, which is completely different type of threading, in which it makes use of "idle threads" wherever possible instead of creating a new thread, thus reducing the requirement to create threads un-necessarily.
Anyway, rather than explain the theory behind the "limit" the short answer is that there is no limit. You are only technically limited by your connection speed. Most sites are around 60k per page, 3 page requests per post, thats around 180k per thread, plus your topic size to upload, multiplied by the amount of topics you wish to send "consecutively" before the rest are qued to wait for a free thread. Generally you just want to keep it around 15-20 threads so you can continue browsing without really noticing any "hit" on your connection.