Well, MAYBE! Be Careful.
Recall that several fellas have spoken to frequency - increase frequency and increase Vne. Increase frequency by adding stiffness. Increase stiffness by adding structure. Increase structure, and increase weight. The process follows that path.
So, let's have a look at the P47 - a ship with almost no Vne. It is heavy. Google an exploded view of this ship - especially the aft section - and count the parts. Looking at that view, you MIGHT get a clue as to how to get your RV up to 300KT. In a dive.
Later, a follow-on version of the P47 was developed, and it's empennage fell off in dive testing: the aft fuselage failed as one engineer had predicted. The pilot survived by following that engineers' instructions (the heavy G forces would oscillate - "jump out when the Gs are negative") - I talked to him about this flight. It was not a fun ride, or so he told.
So, it is not gonna be easy to simply 'increase your Vne'. You WILL be making some parts. Keep in mind that the RV empennage is based on the Midget Mustang design, which is light, and doesn't really go that fast (0-200 power). Parts count remains the same with the RV (3,4,6,7,8), so Vne remains pretty much the same for the empennage.
I would stop there - but you can see where it all leads. You cannot simply fix one area - the aircraft structure
aft of the spar is a system. Be ready to work back from the main spar to, and including, the emp structure, and then you can do your testing for a higher Vne.
The RV wing structure is ready for just about anything you think it can take. I had heard a very fast number associated with the aileron flutter margin, but I cannot speak to the wing itself, tho it is associated with the aileron system and it's flutter number. Seems the RV4/RV6 wing is VERY solid.
I have seen, and supplied replacement parts for, several F1s that have been flown to ridiculous speeds. I will volunteer that beefing up those parts will certainly allow those parts, and maybe the ship, to survive those speeds a second time, but the spar-aft system is not 'fixed'; other internal parts are required to make those speeds 'safe'. I have also been personally involved with one particular flutter event; the part that failed has been beefed up, but which part might fail next?
This flutter discussion is not to be taken lightly: gravity always works.
FYI the P-39 had an aft fuselage problem that let the empennage fall off. More than a 'few' aviators died proving this fault. Big, heavy, external skin doublers allowed that ship to remain in service. This is an example of how to handle flutter, or more to the point - aero elasticity. In this case, the empennage was up to the task, but the structure fwd was not - same as the more powerful version of the P-47. Point B cannot be stronger than Point A, lest Point A fail 1st.
"Gravity always works, and the ground is always hard."
Good luck, and Carry on!
Mark