At which point we're back to square one with Shimano - they basically don't have parts which can be put together to make a clutched 1×10 drivetrain with drop bar levers.
With a short-cage shimano road mech will a 12-25t cassette give me better tension (and less slap) than a 12-30t (assuming each is setup with the correct chain length)?
You'd end up with a bit more chain tension in all the gears except bottom with the narrower range cassette. It's not enough to be a consideration though, get the gearing you want first and worry about chain control as a consequence of that.
At which point we're back to square one with Shimano - they basically don't have parts which can be put together to make a clutched 1×10 drivetrain with drop bar levers.