1.2.10 to 2.55 would be too large as well. I would go from 1.2.10 to 2.0.1 to 2.4.3 to 2.9.1, while walking up i would also then run the corresponding iDrac update as well.  
Is it possible to jump straight from 2.0.19 to 2.7 Bios on an R720 ?or on an R730 from 1.2.10 to 2.8 ?
Question on R720/R730 for Bios upgrade jumps