Iskandar,
I use a bladder on my OS LA .15 engines. Initially I used a pinch type shutoff but have since gone to just closing the throttle barrell to shut off the engine. I do not use a restrictor in the line or a second needle valve. It works very well. You can throttle down about 25% but any more causes the engine to load up rich and you cannot clear it.

The hardest part of the bladder system was sourcing the right materials and then setting up the syringes to fill the engine properly.
A bladder will improve the performance of any engine because it force feeds the engine all the fuel it can take, the limiting factor then becomes how much air you can get into it. Also the running characteristics of a bladder are much better than that of a tank. A bladder is consistent from start to finish of the run whereas a tank will tend to lean out (on a inexpensive .15) once airborn.

The bladder is also far simpler and cheaper than a tank and if it goes bad (burst) you simply throw it away and slip in a new one. I carry about a dozen in my flight box to replace as needed.