It depends a bit on what you call 'efficient'. This article makes quite interesting reading. Although it is probably safe to say that the data from the medieval period needs to be taken with a large pinch of salt there is certainly no massive improvement in yields untill really the 1950s and although there is a gradual upward trend it is pretty flat throughout the early industrial period.
However what did change was that farming became a lot less labour intensive and agriculture becomes much more of a centralised business and much less subsistence based. A big part of this was as much about wider social and economic changes as improvements in agricultural technologies per-se.
It is also interesting to consider the population over this period. Throughout what you might broadly call the middle ages in Britain the population was pretty stable, in fact it too a massive hit during the black death which didn't fully rcover untill the end of the medieval period.
Equally with very few other industries there simply wasn't much for people to do apart form growing food to feed their families and so no major incentive to improve efficiency in term of labour requirments.
As mentioned in another answer the enclosure acts from about 1600 onwards started to change this and made subsistence farming a much less viable option and changed the average peasant from a fialry self sufficient entity into a unit of labour.
So if you were to magically transplant modern mechanised farming into a medieval setting one very likley result would be mass unemployment and probably economic chaos. While landowners woudl be able to produce food with minimal labour nobody woudl have any moey to buy it. See the corn laws to get some sense of the real historical implications of this sort of situation.
Another aspect is that it simply isn't credible to have mechanised farming and agrochemicals but keep the rest of the economy running on medieval technology.
In fact although some of the earliest hints of the industrial evolution came in agriculture it was really quite a late adopter of mechanisation and the real push only came during the second world war when Britain very quickly had to drastically increase production to replace lost imports. Up to this point it had imported a lot of food and let agriculture more or less tick over in fact this period would be a good one to look at to get some ideal of how rapid modernisation of agriculture worked in real history.