I think TEEL is good when you first begin to write essays, but as you develop as a writer it begins to come naturally and you'll just write without thinking about applying TEEL. I still use it for my Text Response (highly recommend TEEL for this) and Context Expository pieces and i'm getting in the 8-9 range, but i don't stop and go now i need a topic sentence, then i need to add evidence then explain it etc. It just flows for me now. With language analysis i tend to just write whatever pops to mind, so my language analyses usually don't follow TEEL and my body paragraphs rarely have a linking sentence.
I disagree with your point about not getting higher then 8/10. It is still possible with TEEL! The difference between mid range and upper range pieces lies within the complexities of your ideas (as you mentioned), the depth to which you explore the topic and the relevance of your ideas.
If you feel TEEL is a bit basic for you, then by all means try experimenting with your style of writing, but i would recommend using TEEL as a structure (especially for Text Response) and just tweek it with a bit of your own style, just to make it unique/stand out from others across the state. But in the end, English is subjective and comes down to impressing the examiners and
standing out from 50 000 odd students.
Hope that helps

What other structures are you thinking of? I only know of TEEL...