in theory if your running prime then it shouldnt make any diff if its on or off, as it pegs cpu at 100% so speedstep wont do a thing.
if having it on de-stabalises you will notice either due to:
-not enough volts at idle and pc just hangs doing nothing (use RMclock to raise the low-clock volts, problem solved)
-not enough volts at 1 of the half way points which tends to crash when loading something, or playing hd films that moderately stress cpu or similar (raise the low clock volts with rmclock, this also shifts up the mid-way volts automatically)
-crap motherboard implementation (nothing to be done)
a lot of the above can be helped if your motherboard has an option for v-core + X(% or absolute) as this usually will raise ALL the voltage levels by the +(%) value. eg rather than setting cpu = 1.5v, set cpu = 1.25v+0.25v the fully idle 1v then becomes 1.25v. and when at full clock, you can now raise v-core further using rmclock on the fly!
(this is all based on the fact RM clock manipulates the VID settings and cant touch any motherboard specific settings, which will be actioned on top of VID)
Edit, pics for greater clarity. pics given are set to maximum power savings as it shows effect the best.
Sisoft power efficiency:
Speed Fan VCore (left hand = set to maximum performance, right max power):
A64 info/speedfan/RMclock combo @2000mhz , note the VID and real v-core - the vd is about right for the cpu @2ghz if it was running stock 1.35v/2400mhz. but mobo is set VID = 1.4v +0.15v (and overvolts a bit) 1.225+0.15 =1.375 + overvolt of 0.05:
A64 info/speedfan/RMclock combo @3000mhz , note the VID and real v-core, VID = 1.4v +0.15v (and overvolts a bit) 1.4+0.15 =1.55 + overvolt of 0.03 (v-regs start to droop at this voltage, by 1.65v there is no overvolt):
