At least a 3C difference if you fill in all the gaps in between the heatpipes and the HSF base, maybe more if you used the dot/pea method too.
Ideally, you want to put a thin line of TIM on in-between each heatpipe on the HSF
then spread that evenly over the entire surface to fill in all of the gaps. You just want to fill the channels level with TIM so that the additional TIM can spread somewhat unrestricted to the edges. So you don't want any TIM actually left on the heatpipes or surface, just in all of the gaps. This is easy to do with a credit card, just press firmly and scrap off any excess.
After that, reapply TIM in-between the heatpipes like in the first step and then place that onto your CPU.