I hear you on those lillies, man! They go crazy in high light/high CO2, and it is difficult to keep up with the trimming. I'm removing at least 4-5 leaves per week. If you had the space, I would suggest that you let it send of a few floating leaves and let it flower. I've done that before. A few days after the flower opens, cut it, and it should slow it's growing down after a while. On the plus side, you could move that to the "open" area you've been "whining" about for the past two-three weeks!
My Barclaya loves my tank, and it should do well in yours! (Hint: it loves root tabs!)
Always top and replant; topping a plant can be done from time to time, but you are usually left with an unattractive stem until a new sprout can be re-formed and grow out. I don't recommend it, but it is a bit more messy and time consuming to do. I hate pulling roots out of the substrate and having a cloud of dust in my tank for a few hours.
Cutting the top off can encourage lateral, bushy growth though....you may have to experiment with this on a plant to plant basis. However, if you let a plant hit the surface, it will often start send out side shoots from it's base as well, which is a great way to multiply stems. If you let the syno's gain in height, they should send out more side shoots from the base. After those gain some height, I would top the main stem and replant, and separate the side shoots and replant those.