High voltage power supply for nixie-tubes
With your current setup, using a buck converter to replace the L78S05C will solve your efficiency (i.e. heat problem).  There are lots of choices to choose from at TI and others.    

The lower profile, less parts count solution would be to use the 5v source to power both the raspberry pie and the Nixie Tube power supply.   There are some 5v Nixie tube power supplies on Ebay (just search nixie tube power supply).     If you are interested, I'm designing a low profile nixie supply that can be powered from 5v and will output 170v (i.e. or 160v as pointed out above for longer lifetime).  The blog is at www.surfncircuits.com and all the schematic, layout, and bom files are at github.  The disclaimer is that the design is finished, but the experimental verification needs to be completed.    I hope to finish it shortly, but the parts are arriving this week.