also don't forget that once your city etc gets bigger/ gains alevel, you can take the food enchantment off your city etc will not get smaller, although you may lose population, but that only matters when you want to grow the city larger again.

like always it depends a lot on your playstyle and the map settings, food can be very important and Fallenchar is not correct with his general statement.

If you want to stretch your empire wide on a big map by settling new and conquering enemy cities, you need fortresses with lots of food to reach the lvl 4 prison and lvl 5 onyx throne improvements to counter the city count penalty. If you want to create a conclave city that utilizes its essence yield to the fullest and therefore becomes a research and mana powerhouse, you need lots of food. If you need to maintain a big army financially and if you want to counter low production locations in your empire, you need lots of food to reach the higher level town improvements.

So food is worth a lot even if you play on a big map and if you want to establish and maintain a vast empire.

Corrected that for you. The first correction is meant as a joke - obviously, but feel free to view it otherwise  -.

Anyway, it would be interesting if someone with math-skills could calculate when it would be more beneficial to build another pioneer or to keep growing to the next level, under various conditions (nr of cities, type of city, map size, turns played, etc)

Focus on food as in building those improvements first? No. You only get 3 growth from food max and it will decline as population grows. No point in getting those improvements until you need them. (remember you use population for pioneers early on, so it will be very slow in the start) A high food tile is good for spamming pioneers, not becoming a big city quick. 

Focus on food as in reaching city levels as soon as possible, yes. Use consulates(outpost improvement), spells and Wildland heads to improve growth, when you get 6+ growth your cities should get there very quick. 

A high essence, high material city is the optimal for quickest lvl 5 city. Use the essence slots on food and growth enchants, get the food buildings up faster and you have the ability to get consulates up as well for growth, all that makes it far easier to get the precious lvl 5 on your city.

Ah, this was the key piece i was missing!

That makes growth a lot stronger than i thought, because while buildings can generally get me another +1/+2, growth can get me a much bigger bonuses.

Perhaps you haven't played that many games, but yes it is true, the passive bonus exist, it's there since vanilla FE (FE v1.00), it's even there since Elemental War of Magic (if you still remember this game).

It doesn't take that long to level up the city to high level, things has changed since FE, in FE you need high food yield beside high growth to reach level 3 or even 4 fast, in LH you just need growth, food is not as important because the maximum population needed for level up has been lowered (the change log posted by Derek doesn't lie), and it's easier now to obtain food (especially for town type city).

That being said, i'd still advise not focusing in food because of that, food is easier to increase now than in FE, however growth is still the same problem, but like in FE there are several ways to solve this, the spell sovereign call being the easiest and fastest way.

A balanced focus with the least in food is useful for most situation, but in the end it depends on individual strategy and play style.

Also this is just for info, the passive bonus in LH is not as useful as in FE, because in LH when a city reach lv 2, the type of city now give much more bonus than in FE, example is fortress in FE only give +1 level to trained unit, now it give many bonus beside that. However the passive bonus (not town specialization bonus) is massive when a city reach level 3, even more so for level 4 and 5, from real experience i'll take conclave as example, at level 2 a conclave without any modifier for research produce 10 research, at level 5 it can reach 40 research, with modifier, the modifier will give higher bonus because the base number which it modify increase so much (a conclave with 100 research is not a myth). In FE, +x % research building like "sage" is not as useful as the flat version increase like "school", however in LH it works the opposite, the +x% being more useful now because city in LH level up much faster than in FE, which is why food is not as important in LH as in FE.

My last Ridiculous game (medium map) lasted 234 turns, something like that anyway. That was the longest game I've played, and the only time I've got to a level five settlement. If I don't think I'm clearly winning after 100 turns, something has gone wrong. To put it another way, by the time I get a level four settlement, the game is practically over. So while bigger settlements are nice, they're not really something I focus on.