Some issues about upscaling and neat video

SD has as little as 1/6 the information that HD does (at its highest resolution) so I'm not surprised that you're getting a poor image when you do this.
Boris has a plugin called Uprez ($100 U.S.) that may be of some help.
A free trial is available.
http://www.borisfx.com/units/upRez.php

accept it. nothing more you can do. upscaling is nothing but adding pixels. someone has just asked that a moment ago. you can't expect anything, any plugin would magically reveal pixels which are not present on original footage. upscaled from sd to hd has more "assumed" interpolated information than original information. no wonder it is blurry. it will be. it should be.