A solution is to make an (advanced) option to change the tab-width independent of the normal tab-size. Per default, the tab-width would mirror the tab-size so pressing tab (in non-emulation mode) would insert a tab character, but as an advanced setting you can set tab-width to 8, while size is 4.... but I don't blame Allan if he doesn't want to implement something like this.. It could be a mess :-p.
I have no idea why emacs uses this method (in its most usual setup), but maybe to keep tab-width at 8 so it works with older apps and e.g. HTML and such? But then, why not only use spaces and never tabs like when TM is in emulation mode?... weird :-p.