I agree, at high speed, even small particles will impact with the force of a 16 inch shell.
If my math is not too badly off, if your ship travels at 3.000 km/s and hits a 1 gram particle, that particle will impact with a force of 4.5 billion joules or about 1 ton of TNT. That´s nothing to lough at!
E = 1/2m x V x V --> 0.0005kg x 3,000,000 m/s x 3,000,000 m/s = 4,500,000,000 j
1 gram of TNT equals 4184 joules --> 4,500,000,000 j / 4148 j/g = 1,075,525 g or about 1 ton of TNT
Yes, even in a dense nebula, particles of that size will be very rare, but even a particle a thousand times smaller will hit like 1 kg of TNT and those will be a lot more common. I sure wouldn´t want to go through something like this in a tincan at anything but dead-slow speed.