an opera tab isn't an MDI child window so your comparison is completely moot. You need to compare windows to windows... IE hits the desktop limit at roughly 30 windows (NOT tabs).
Using MDI windows is not "abusing the heap", it's merely using MDI windows. mIRC is not going to change from using MDI child windows in your lifetime, it's one of the features that practically defines the UI, so there's little you can do but increase your heap.
