15 AoB climbing turn work fine and does not significantly diminish climb performance.
Other turns 30 AoB. you are very unlikely to stall from high Aob on base and final. you are much more likely to have issues if you use a shallow turn (or terrify poor luckless students that this is a maximum angle cos anything more is potentially 'dangerous').
A shallow turn, say 15 AoB, when added to a TWC on base means that the student may get pushed past the RW ECL and attempt to tighten the turn WITH RUDDER (cos more that 15 is dangerous remember). classic scenario. skid causes drag and sink, student raises nose to attempt to get picture right and treads a bit harder on the rudder, are you all following me?, there is a need to oppose the rudder with opposite aileron, so keeping low decelerating) wing up with aileron, nose starts to fall despite back pressure, low wing continues to descend despite full opposite aileron, ground getting closer, and closer faster, pull back really hard .........
15 works for oval but 30 is better for regular pattern, enabling sufficient time established with wings level to assess, adjust and confirm accurate position and approach profile.
Doing a 'derry turn' at circuit corners is fun but not for the pre solo student.
personally i like oval, but i do what is the 'normal' thing where ever i go. the "standard" pattern shape and altitude is there for a reason, not entirely sure what that is, but it does enable the instructor and student to determine if the standards specified in the instructional objectives are being met.
HD