Leaking new vents?

what is your pressure set at?

At those pressures I wouldn't expect any hissing, and even less so at the start of a call for heat when the radiators are still getting heated by the steam. Closing off 1/3 of your radiators, though, will make your boiler even more oversized than it already likely is and that's not advisable.

Blanket or shutting them off it still oversizes the boiler.

Don't worry about efficiency. The BTUs are going into your house, there's no other place for them to go

I see several problems here. You don't need any "minimum pressure" to close vents. They operate strictly on temperature.

Your boiler is cycling on and off rapidly on pressure nearly all the time. If the radiators are full of steam to their ends, your boiler is terribly oversized.

Between the atrocious piping of the boiler and its oversizing, you are dealing with a very high steam velocity into the radiators. This is exacerbated by using very fast (large) vents. This negates condensate drainage and causes wet steam to reach the vents. This wet steam makes the venting noisy. Wet steam also cools the vent's bi-metal element preventing it from closing tightly.

Putting a slower vent on the larger radiator along with increasing main venting may help quiet your system. Substituting a bi-metal vent (MoM or Gorton) with a sylphon vent (Hoffman or Vent-rite) could possibily help too. They tend to close at lower temperatures and/or not open as quickly.

I'm afraid, though, that such oversizing and mis-piping this boiler will have these symptoms.

Yes your boiler is rated for 391sqft but that is to be compared directly to your standing radiation, not the radiators + piping. The boiler rating already accounts for the piping estimate. So you are almost 50% oversized with all your radiators open. In my opinion you would be better off with slowing some of the radiators down getting a little more length out of a cycle to give the house a chance to heat up before all the radiators are filled and you start cycling on pressure.

If you are really that oversized maybe think about adding a timer that will pause the firing during the call for heat. My system was oversized by ~40%, but it was an older home with older windows that probably needed all of the pickup factor, plus it had a Cycleguard low water cutoff that would pause firing during a cycle. That Cycleguard delay mitigated the short cycling such that it was only really noticeable on the coldest days/during prolonged cold snaps. You could add a control that does the same thing and dial it in more precisely. I also considered converting to the next smaller sized burner which would have helped alot but I'm out of that house now and someone is probably tearing the steam out anyway 😪

My oversize situation was not too severe. Check out this thread for some suggestions:

That could potentially be a concern, but it can't be too big a deal…most manufacturers do the same thing (remove a burner) when they go from one size of x sections down to the lower size of x sections.

For example, my boiler the 63-03L is just a 63-03 with one less burner

So true, and I tried to word my post not to infer that your suggestion isn't a good one. I just wanted to mention that there are considerations that must be, eh, considered. When manufacturers market a boiler with different burner configurations they have done that considering for you and are confident that the option is worthy of their "good name".

the rate of a given radiator filling has a lot of variables but that sounds reasonable

Forgot to mention, the pipes between the header and the main get steam-hot after about 2 minutes.

And this can vary a lot depending on how much time has passed since the last boil, but in the winter it is probably boiling about every hour anyway so will never take long to start making steam.