If you have gone from a well worn shoe to a brand new shoe, that in itself could cause some teething problems. Have these shoes been tinkered with between 13 and 15, e.g. a reduced heel drop. If you look into the reviews of the 14 and 15 it may reveal an unwanted change
I used Sports Tours International for NY this year. I can't say they added much to the occassion. They organise flights and/or hotel, marathon entry and airport transfers. Other than that I was a little underwhelmed by their role. It seems like a licence to print money.
New York Road Runners pump out all the info needed. One or two things that were promised by STI didn't materialise such as breakfast being available before boarding the coach to the start. I'd second guessed that was going to happen. I'd also complained about one of the reps. The rep I spoke to said they would take the complaint forward. Never heard anything back.
Unless you get direct entry, you have to use an operator such as STI. I would use them again if they were cheapest.
I've ran in a 209 event in the UK. Was well organised. Not tried any overseas events yet.
Yasso 800s were more of a marathon time predictor than a proper VO2 session. Run then hard and as evenly times as possible; Yasso advises taking long recoveries the produce 10 x 800 reps. For what purpose were you doing them? If it is a VO2 Interval, then I would stick with a 400m jog recovery, or maybe 1:30 - 2:00 mins recovery however best suits.