Right, but the internal memory used is what documentation calls (wrongly) the ROM memory, which holds the applications and operating system. Not RAM, which is the operating memory.
The point is that, unless you have that ROM almost all the way filled up (100+ apps, for example), your phone's performance won't much be affected by this memory. If texts and cache occupied some of the RAM instead, I would agree that clearing cache and texts would speed up the phone. Clearing cache should make your browser, albums, and other things slower, in fact. Clearing texts might make your Messaging app (I know this is true for Handcent) perform faster. When I open a Handcent thread, for example, all of the 231 messages (or however many) in that thread open all at once. It's how I have it set up. This, however, makes it much slower to load each thread.
If Handcent stored the visible message thread in the cache, instead of reading from your internal Messages and rendering the thread every time, from scratch, Handcent would perform more quickly, not less. Clearing this cache would force it to re-load the thread again. Which is slower, until the cache fills back up.