From what I've seen, the problem with corally tubes is that they have a very loose fit. There's a too large gap between the parts.
I lathed my own tubes for my Pro 10 car. There's very little play between the parts, and they work really well (I smoked the German top drivers with it), without slots in them.
However, if you use tubes with a tight fit, don't use corally damper syrup. It's too firm. Use Losi hydra fluid or CRC tube fluid instead.