Is there something like an explosive effect I can achieve?
Given the handy metre-cube limit, lets think about the energy required to raise the temperature of that volume of liquid water at ambient temperature (say, 20 degrees C) to 8000K. Lets say it is a nice round tonne of water. Specific heat capacity of water is 4181J/kg, so you need about 335 MJ to bring the water up to the boil. Next you need 2.2564MJ/kg to convert that water to steam, which gives you another 2-and-a-bit GJ. The SHC of steam is more like 1996J/kg, so the next 3000-odd degrees will need another 6GJ.
edit: as cmaster rightly pointed out, once you get to about 3000 degrees, water disassociates into hydrogen and oxygen. The average energy of an H-O bond in water is 461kJ/mol or 25.6MJ/kg, so that'll need another 25.6GJ* to fully disassociate the entire tonne (also note that all those excited and solo hydrogen and oxygen atoms are gonna want to hook back up again, and that's gonna produce an exciting effect too!).
Now, the SHC of monatomic gasses is nice and simple and related only to their atomic mass (because there are no complicated atomic bonds to wiggle about). Hydrogen gets a high 12kJ/K/kg, and oxygen gets a slightly more modest 750J/K/kg. For the remaining 4700-odd kelvin, the hydrogen will need 6.2GJ and the oxygen will need 392MJ. This is about half the energy needed if the water molecules were still intact. The total energy is therefore about 35GJ.
The "tonne of TNT equivalent" energy measure used for nuclear bombs and the like is defined as about 4.184GJ, so the amount of energy you've just poured into the system is the equivalent of a 8.3 tonne bomb. This is is in the same ballpark as the the 11-tonne yield of the GBU 43/B MOAB (which looks like this in action [youtube link]) or its predecessor the BLU-82 Daisycutter which has a slightly more subtle yield of about 5-7 tonnes, shown in action here:
Now, I don't think this magical instantaneous superheated-monatomic-gas transition is quite the same as explosive detonating (I'm carefully ignoring temperature/pressure relationships, because I'm lazy), but I would absolutely not want to be anywhere near it. That incredibly hot gas products will be under formidable pressure when it forms, and quite a lot of that energy you've poured into it will be released in the form of vigourous and rapid expansion. Boom.
This could perhaps be even worse if you picked a material which took even more energy per unit volume to heat this much. There are a bunch of factors at play here, especially given the energy sucked up by molecular disassociation, so it isn't clear what would be the most destructive. Some form of stone is probably the winner here, but working out by how much it wins is quite tricky and left as an exercise to the reader.
At the moment I am thinking about capping the maximum temperature at 8000°K. What would change if that was not the case?
At a "mere" 8000K, you're probably below the temperature at which a significant amount of the hydrogen and oxygen in the aforementioned water explosion would be ionised. Push the temperature to over 10000K though and you'll need another 1300kJ per mole of hydrogen or oxygen to knock off some electrons and make a nice singly ionised plasma. It might quadruple the total energy released in the blast compared to 8000K (but I'm too lazy to work the actual output).
But what if we went even higher? Arbitrarily high temperatures and a magical effect that propagates at lightspeed. What could possibly go wrong?
If you had access to the right kind of fuel, this sounds like a recipe for Inertial Confinement Fusion.
Now I want to weaponize this ability, and need some guidance for the implications.
My recommendation is to make it much less hot, and affect much smaller volumes of material. I mean, seriously, just bringing stuff up to a few hundred degrees C is enough to start fires, deform or melt soft metal objects, render other objects unusable or downright dangerous (would you want to be wearing a helmet that has suddenly become 500K?). Your enemies would have to approach you nude (but for some asbestos sandals, perhaps) and emptyhanded. That seems reasonable enough, compared to town-flattening explosions.
* I'm not 100% sure on this; the bonds might just fall apart because they've already had enough energy pumped into them by heating, and there might be no additional threshold to overcome. If this were the case, the yield drops by about 2/3rds, but still leaves you with a respectable couple of tonnes. Also, the disassociated H and O will vigorously recombine to release that much energy afterwards, contributing to the fireball if not the blast.