Do 6/4 favs in lower grades win more often (in % terms) than 6/4 favs in higher grades? May be a better question (if there are enough examples to make it worthwhile). If roughly the same – then lower grades can not be "easier", but your stats do seem to suggest it is no more difficult.
I checked to see how exact 6/4 shots performed over the last 10 years and here are the figures:
Grade 1 & 2: 84/234 (35.9%) -10.3%
Grade 3 & 4: 246/714 (34.4%) -13.9%
Grade 5 & 6: 450/1189 (37.8%) -5.4%These too looked quite skewed towards lower grade races, albeit with unreliably small sample sizes (in my opinion) at the top end.
I decided to next check on all horses between Evens and 2/1 inclusive (not necessarily favourites). The obvious caveat this time is that I wasn’t comparing exactly like-for-like as there would not necessarily be the same percentage of
exact
6-4, 7-4, 15-8 shots etc etc in all three groups. However this time the control size was big enough to give the stats plenty of credibility:
Grade 1 & 2: 747/2158 (34.6%) -10.7%
Grade 3 & 4: 2368/6569 (36.0%) -7.0%
Grade 5 & 6: 4399/12030 (36.6%) -5.5%If one is prepared to take the stats at more or less face value, the implication is that if you’re going to bet at the sharp end of the market, you would be better off doing it in lower-grade races. I see absolutely nothing in those figures to support the widely-held opinion that higher-class racing is somehow ‘better’ for the punter overall, although I understand what you say about how it suits your particular analysis.
Mike
Many thanks Mike, that’s food for thought.
May be I should give the poorer grades another chance.
Not so much money is wagered on a gaff race than Grade 1. Therefore, the
percentage
of over all race stake money taken for a "steamer" is far greater. So a horse will shorten far more for the same amount of money wagered on it.
In gaff races the prize money is very low and the only way trainers/owners are able to "make money" is a betting coup. With perhaps those being backed "more likely" and those not being backed (drifters) "less likely to show their form". Which may in tern mean those at the front of the market having a greater chance.
Obviously there is no point in backing blind and making a -5.5% deficit. The problem (as ever) is knowing which horses are the ones likely to be backed (value).
I was once asked if I’d like to put my name forward (no more than that) for doing "Spotlights" in the Racing Post, which includes making the "betting forecast". Obviously those just starting the job would need to work in poorer grades. Where as my own betting forecasts (for betting purposes) on Grade 1,2,3 and even 4 were pretty good. My trials for Grade 6 proved impossible (way out), so I didn’t take it further. 
May be I didn’t give it enough of a chance Mike, but as Drone says, specialisation is key.
However, your figures do strongly suggest Grade 5 and 6 aren’t the impossible races I once thought they were.
On the other hand…
If those at the head of the market don’t do as well as you’d expect in Grade 1 and 2; does that mean mid-range/outsiders do better than you’d expect in that grade? More big priced winners = "easier" profits Mike? 