The E6700 ES I have atm is currently sitting under my phase with the evap @ about -48 (K-Probe) and about -40 in the BIOS which is pretty decent I think personally.
Defo keep it, 4.5Ghz is a little ambitious though I have to say, as we needed LN2 for that or cascade @ -84. Doesn't mean I won't be trying it though
Boardy