You can exceed $417K on a VA, but you must be able to put down some extra cash. There is a formula for how much. 25% of the amount over $417K.
For $800K with full eligibility, you would need to put down $95,750 (~12%) to obtain a VA mortgage.
If you are in a high cost area, the $417K goes up to $625K, so you would only need to put down $43,750 (~5.5%).
You will not want to do a conventional loan for that amount, most likely, since it is considered a jumbo loan. It will have a higher rate and likely require PMI even if you have 20% to put down.
Side note: putting down at least 10% on a VA gets you the lowest possible funding fee. If you are a disabled vet, 0% rating or higher, then the funding fee is waived.