Isocapnic training is more valuable for higher frequency breathing, you do not need to train respiratory muscle strength. One benefit though, would be with something like a Spirotiger you would get a read-out of your respiratory volume and you could use that number to train breath volume and monitor exhale 'quality'. With something like a Breathe Way Better, you could use different bag sizes to measure expiratory volume. But the caveat is that if all you're looking to do is improve the function of the respiratory muscles then you don't need an isocapnic device. You could just do self-monitored breath work with a focus on volume and you'd get the benefits you're after.