No, not all T5 bulbs have a high output. Only T5 bulbs labeled "HO" are high output bulbs. T5 HO boast higher wattages and lumens, making them perfect for industrial and commercial high-ceiling spaces. For example, regular T5 bulbs produce about 2,900 lumens while T5 HO bulbs produce 5,000 lumens. T5 HO bulbs also need their own ballast to function properly.
T8 LED bulbs can go inside the base of the T12 fixture. T8 tubes most likely require removing or bypassing the T12 ballast to work. However, T12 ballasts aren’t commonly compatible with T8 bulbs for operation. Attempting to use a T8 in a T12 ballast may lead to bulb failure.