I've made a short test if it's worth to truncate the SHA1 URNs to 6 digits.

The raw string length saving is 81% but with compression is the saving only ~50% compared to a complete and compressed SHA1 URN (or other way around; for only 50% extra you get the real deal with 100% accuracy) and we're talking here about 26 vs. 52 bytes. I cannot estimate if it makes a big difference for UPs as it sums up over time and load.

Note: the calculations include a fixed gzip header. Without this header are the numbers a bit better (up to 70%) but it's still less than what you save by truncating the string. String compression is quite effective.