You might want to dig through old issues of ENTERPRISE INCIDENTS magazine. There are a couple that have concept art by the original (fired) VFX team on TMP. Their concept for the warp effect involved a sustained ripple between the inboard grills of the two nacelles, plus a sort of bubble around the ship that manifested primarily as a distortion of the star background.
It sounded both complex and subtle, and would have required shooting two differently sized Enterprises (one of them a black miniature with fiber optics strung between the engines) lined up in perfect synch to get the inboard effect to matte itself as the ship moved through the shot, but if they'd pulled it off it might have been awesome.
Are there concept drawings/renderings of this somewhere? I would love to see this idea realized. Just to see it.