Changes in filtered 2.5

I believe that with a filter, two 50% is enough, as long as you vacuum the gravel.

It allows the ammonia to build up all week, instead of only half a week. Full changes are also more stressful than partials.

Not true, Liquid. In a 2.5 there is no cycle anyway - less than 5 gallons doesn't really hold a cycle. In over 5 gallons, which has cycled, even a 100% water change won't remove the cycle. Very little of the beneficial bacteria is contained in the water.
What will kill a cycle is replacing or over-scrubbing your filter, which is where 90% of the good bacteria is.