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In terms of return on investment, I would have to say it's a tossup. However, there are other benefits to insulating the mains — and efficiency is probably the least of them. The most important is that insulating the mains increases — sometimes substantially — the speed with which steam reaches the length of the main from a cold start. The means that it is much easier to get a good balance for the heating in various parts of the building, and it also means that the total run time of the boiler will be less. How much less? Hard to quantify.

There is also the benefit of reducing heat loss from the mains, of course. Whether that is a plus or an actual disadvantage depends on what, if anything, the basement is used for and how well it is connected to the rest of the structure. I warn you that there is a rather lively debate on this aspect…

In my opinion, insulating the mains is worth the cost — but some of the benefit will not be apparent to strict bean counting.

In another topic. The crossover trap and rip which you note are essential to correct operation of the system. Assuming that the crossover trap is working, the sole main vent at the boiler is ample. The other main which goes to the large radiator (sometimes referred to in old literature as a condensor), and it was thought to help speed up steam flow to the end of the main) should also have a crossover trap. It would worth figuring out where it might have been, and replacing it. Without it air removal from that main will be dependent on venting through the radiators. This is adequate, but slower than it might otherwise be — and limits the extent to which you can throttle radiators on that main, should you wish to do so.

I wouldn't be at all surprised to discover that this system was intended to operate on less than 1 psig, possibly as little as 8 ounces gauge. Depending on the radiator fittings (inlet and outlet) it is quite likely that it will operate more evenly and more efficiently if the pressure is limited to 8 ounces gauge.

The system should not water hammer at all. Water hammer is very rarely to be expected, but is almost always related in older structures to sections of main or runouts which have inadequate — or even reverse — pitch. It would be worth investigating and checking the full length of all the mains and runouts to ensure that they are, in fact, still pitched properly. Pay attention to sags.

And to this before you insulate!

Since it is grossly oversized then insulating your pipes will make the short cycling more of a problem. You are already overpaying for the heat you need, what is a little extra insulation going to do for you?

After you come to replacing that boiler, years from now, with the proper size boiler, then I would look into insulating the pipes then.

Just an opinion, not based on facts.