I'm on my final 90 day visa for my current Non-O visa. Yes, the final 15 months.

Could someone kindly enlighten me as to what the financial or time advantages are of a non-O visa versus a retirement extension?

As I see it, if I obtain another Non-O, I'm up for a border run every 90 days. If I obtain a retirement extension, salting my savings into an account so that everything + pension totals ฿800,000, I understand that the 90 day visa run can be done at CM Immigration. (shudder)

Is my summary correct or not?

Just now I feel that the 90day visa run can be made to a neighbouring country, giving me an excuse to visit elsewhere, and no real desire to visit Immigration.

Guess it depends how easy it is to get the non-O. I do the same, but I travel to the UK twice a year and can get from there extremely easily. I never stay in Thailand longer than 6 weeks so never need to do visa runs. So it suits me just fine.

Although you can do your new entry runs to nearby countries you are not likely to be able to obtain another visa there (unless you have more than retirement as a reason for issue). There is no visa run required with a one year extension of stay for retirement. That 90 day visit is for reporting address and can be done by someone else (in Chiang Mai) or by mail at most places. You are not likely to obtain a multi entry non immigrant O visa at most Consulates - you would have to apply for a non immigrant O-A visa with medical and police reports (but you would get a one year stay on entry rather than 90 days).

and also important with a retirement visa with M entries,after one year you can go out the country before the" end "of the visa,and you will receive one year extension with every 90 days visit to the immigration office(TM47) for extension,if you need go out the contry in this second year,just a re entry permit(single of M)is necessary,for the single re entry (cost 1900 bath).

In two words,one year visa Non OA M entries,can be good for 2 years and no too difficult a trip to Lao or Myamar for one or two days.

Actually, since the OP is nearing the end of the final permission to stay of his O visa, he can simply go to CM Immigration during the final 30 days of that permission to stay, demonstrate the financial requirements and obtain a 365 day extension due to retirement, without having to get a new visa.

Incidentally, despite all our whining, it's not that hard to do 90 day reports in Chiang Mai. Usually takes less than an hour and it's free. You can pay a visa agent 1500 baht/year to do them for you. Mail-in 90 day reports aren't an option in CM, but there are rumors they'll accept mail-in for people who live a long way away from Immigration.