You know, I never used to admire clumps or groves or rafts at all until I started finding examples in Japanese exhibitions and nurseries (though technically it was all through blogs, but whatevs). Likewise, they've become one of my favorite styles, especially with maples. I notice myself getting more excited about a beautifully styled clump than any other style, unless it's something mind-blowing. But anywho. . . . I think it's a terrific idea to dedicate a good chunk of your time to making clumps. I have 500 kaede seeds stratifying in the fridge right now, so perhaps I can try to parallel your project with them instead of momiji.
As for this tree, I totally agree with you. I think anything taller would ruin the image. I plan to grow outwards and work mostly on ramification. Only time can really bring out the beauty of age in the tree, but I'll wait.