Rule of thumb overpowered ANY plane is better then underpowering it.
Reality is that our RC aircraft are all waaaayyyy overpowered in the first place. We enjoy a power to weight ratio that the full scale pilots can only dream of. Look who is doing 3D. It's the RCers. I like overpowered planes. I have many overpowered planes, but trainers are NOT the place for being overpowered.
I have a Kadet and because of the weak engine on it OS .40LA its really hard to take of and perform simple maneuvers.
You did not say which Kadet (MKII, LT-40, Senior, Junior, Seniorita ect.) I'm guessing that you had one of the bigger Kadets (LT-40 or Senior) that you had the 40 LA on. If this is the case, then I agree with you that the LA is too small an engine for these planes. I would say that for the Kadet MKII, an average 40 size trainer - the LA is perfect.
I will admit that the OS 40 LA on 5% fuel does not have much power. On 15% it has great power and a tick over idle. This is where I make my point about getting to Solo faster. Ball Bearing engines (which I love, but not for trainers) swing a bigger prop and idle faster. This all comes down to having too much thrust during landing.
P.S. Nice pictureof those trainers really nice trim. Kit or ARF?
The plane is the Hangar 9 Solo Sport ARF. It sells for $100. I have been instructing with the middle one for 3 years with a 5 year old OS 40 LA. Before that I instructed for one year with a Kadet MKII and for three years with a Midwest Aerostar.