Pitch doesn't say much nowadays, as it includes the thickness of the seat. So slimline seats that are an inch thinner would indeed offer an inch of extra leg space, at the same pitch value.

However there are huge differences between slimline seats. Back support is a big one, but location of the seat pocket is too. When your knees do touch the seat in front of you it makes a huge difference if that is against a plat piece of plastic, a seat pocket, or a mechanical part.

For example the old Air France A32x seats were horrible, as there were aluminium mechanical parts (knobs, nuts, bolts, whatever) right at the location where your knees touch the seat in front. However nowadays that is much much better with the new seats.

In general I try to avoid KLM's 737's if I can nowadays. Compared to the A32x the seats are quite narrow and the EC debacle is still bothering me as well - I'd like a seat towards the front without having to pay extra. For example AF does offer that.

Those have been replaced not that long ago right? Perhaps 2-3 years ago?