Well the only way to 'upgrade' the DI system is by overspinning the HPFP. The VTT two shotgun still overspins and there have been numerous issues with their kit. Failing Helix and other overdrive products etc. Having to add braces and billet parts because of harmonic issues spinning a pump past it'd design limitations is just stupid.
AFAIK drilling out the HP fuel rail only gives you a small bump in headroom and then you need to install an quality in line fuel filter. Just drill out either rail. I have not seen any documentation or reason to believe one is better than the other, especially when drilled out. Remember, VTT used to charge $400 to drill out your HPFP rail which is a 30 second job!
Like most going to PI makes the most sense. Especially if you want to run full E85 and not mix at the pump which is PITA, especially if E85 aceess is hard to acquire.
When you factor in the cost of a HPFP overspinning product + a new HPFP PI makes even more sense. Or, overspin your 100k HPFP and see how long it lasts. Hopefully as long as the Helix etc. HPFPs and most pumps in general do not like spinning 30% past their design output for obvious reasons. If this was not an issues we would see 50% or 100% overspinning. There is a reason they do not.
A PI spacer + AIC is still the cheapest solution and you get the added benefit of never not having to clean your valves and the all the headroom you will ever need. At the end of the day the N54 injectors are always a hard limit in terms of injection and no high HP build is ever 100% DI based, and for good reason. DI upgrades are a band aid.
There is also the theory you should never run your fuel system past 80% of capacity LP or HP, especially HP. If you need the 3-5% the HPFP rail gives you it sounds like you are running on the ragged edge of your DI capacity anyways which is a recipe for disaster.
TL: DR Get PI Plate + 750CC
edit: I am not aware of a single post when someone had an overspun HPFP, then went PI and said, "Gosh I wish I went back to overspinning my HPFP and back to DI!"