I was about to suggest running with a +Spe max invested set with minimal SpA investment to insure speed boosts on kills but... I didn't realize just how high its SpA actually was until I tried to do a calc >.>
Glad I actually tried that one out before I opened my big mouth and made a fool of myself in my virgin post. Even with a negative nature and absolutely zero investment, his SpA is still too high. What a hero.
Well, so that idea doesn't actually pan out, how about a Normalium-Z Hyper Beam set?
Xurkitree
Item: Normalium-Z
4 HP/252 SpA/252 Spe
Modest/Timid
-Hyper Beam
-Zap CannonDischarge/Thunder/Thunderbolt/Volt Switch
-Energy Ball/Grass Knot
-Hidden Power Ice/Dazzling Gleam
I mean, most of those moves are negotiable, and it still gets walled by A-Marowak, but what doesn't at this point, honestly? Discharge trades 10 power for potential paralyzes, potentially allowing him to outspeed pokes that he wouldn't otherwise, but you can run thunderbolt just as easily, or thunder in a rain team. Energy Ball or Grass Knot cover the same types but different pokes more or less effectively depending on weight (y'all know how it works). HP Ice and Dazzling Gleam are just bonus coverage moves.
252- SpA Xurkitree Breakneck Blitz vs. 248 HP / 92 SpD Amoonguss: 261-308 (60.5 - 71.4%) -- guaranteed 2HKO after Black Sludge recovery
252- SpA Xurkitree Breakneck Blitz vs. 0 HP / 0 SpD Garchomp: 278-328 (77.8 - 91.8%) -- guaranteed 2HKO
252- SpA Xurkitree Breakneck Blitz vs. 244 HP / 200+ SpD Gliscor: 221-261 (62.7 - 74.1%) -- guaranteed 2HKO after Poison Heal
252- SpA Xurkitree Breakneck Blitz vs. 252 HP / 112 SpD Hippowdon: 276-325 (65.7 - 77.3%) -- guaranteed 2HKO after Leftovers recovery
252- SpA Xurkitree Breakneck Blitz vs. 248 HP / 8 SpD Landorus-Therian: 289-341 (75.8 - 89.5%) -- guaranteed 2HKO
252- SpA Xurkitree Breakneck Blitz vs. 0 HP / 0 SpD Excadrill: 172-203 (47.6 - 56.2%) -- 82.4% chance to 2HKO
252- SpA Xurkitree Breakneck Blitz vs. 0 HP / 0 SpD Manectric: 367-432 (130.6 - 153.7%) -- guaranteed OHKO
252- SpA Xurkitree Breakneck Blitz vs. 248 HP / 8 SpD Landorus-Therian: 289-341 (75.8 - 89.5%) -- guaranteed 2HKO
*Edit: turns out I forgot to switch to a more beneficial nature after checking to see if you could raise Spe using a negative SpA nature, whoops.
If you can get a good switch on something that doesn't want to take a STAB thunderbolt or whatever doesn't resist Xurkitree's more common coverage, you can do a pretty good chunk to anybody who tries to make a predicted switch-in to absorb an attack, effectively neutering most of the pokes that will switch in on it. This might be especially effective on pokes running Lightning Rod in the hopes of getting a boost on a scarfed/specs set.
I think there's always merit in running something unexpected, because it allows you to take advantage of complacency. Besides the Breakneck Blitz, the set is more or less the same, so even if you switch Xurkitree in an out occasionally to harass, there won't really be anyway for your opponent to know if you happen to be running this specific variant, if they're even thinking about it at all in the first place.
I mean, if you can dismantle a priority target or seriously injure a wall with no reliable way to recover, you don't even need to stay in if/when you get the Beast Boost, because Xurkitree will have already done its job at that point.
Just food for thought.