I was on the same boat under 1k paragon. Most players that clear GR90 smoothly are paragon 1.5k and up, and that's with ANY build. That's just something to consider. It's a strong build and offers something different. Completing 80+ is more challenging because it's a tanky build. Personally, I like this build because it's not CDR dependent. It's more engaging than running around waiting for LotD.
Well, I tried this build precisely as it set. I tried a simple GR80. I wasn't dying, but it took forever to kill stuff and elites were a no-go. I subbed BoS for PE, and that seemed to work a bit better. I did not even come close to dying, so it is tanky, but it took me 20:28 to clear that simpel GR80 which is no good. I tried my simple bone storm build and completed it in 8:36 without dying. This build is tanky, but does not have the DPS to ever do GR90, at least not for me as I am under P1000. Great defense, NO OFFENSE!
Other players will prefer high dps builds because they want to beat timers. This build isn't made to race the clock but it's a good build and can get you to 80's in no time. Clearing 80+ without issues requires a bit of paragon and sheet damage.