Feature size prevents computation from being fast, even if you could tweak it to create "organs" that did distributed processing. Basic limits of what cells can do would constrain the clock speed of any particular compute component to ~6khz or less, or about 100x what they can do naturally.
A mycelial mat, coupled to a PCB, with a few genetic tweaks, though, could allow traditional computers to make use of incredible networks that have trillions of intersections; if you could somehow elicit the algorithm underlying human cortical networks, edit fungi to mirror that, you could get a potent AI hybrid device, about the size of a doormat, as thick as a paperback book, but it's not something where a little chip sized chunk of fungi is going to be useful for a wearable. You might do interesting things with robot sensors, if the electrical signaling translates to touch, maybe a thin network of mycelium over a robot could act as robot skin/touch/pressure sensors?
Bravo, science, fun stuff. We're going to get autonomous cyber hybrid mushroom robots, like a "Last of Us" and "Terminator" hybrid.
[from arxiv description] Fungal electronics is a family of living electronic devices made of mycelium bound composites or pure mycelium. Fungal electronic devices are capable of changing their impedance and generating spikes of electrical potential in response to external control parameters. Fungal electronics can be embedded into fungal materials and wearables or used as stand alone sensing and computing devices.