Hydro in the setting of sepsis is quite concerning and not always due to a stone. UPJ obstructions, ureteral stricture, obstructive mass are some of the other causes, and all would require decompression in the setting of acute illness. But whether is 10% or 30%, what is an acceptable rate of failed source control for you? Especially since an ultrasound has 0 risk and if they're ill enough to require admission won't affect your dispo.
I agree that using 2 SIRS criteria + UTI is overly stringent, but thats where your clinical judgement comes into play. 20 year old female with fever and HR of 105 (while febrile) is technically septic but I wouldn't image. 60 year old with tachycardia and hypotension of suspected urinary origin should be imaged. Basically if you're considering the unit they should have abdominal imaging. I've seen patients basically dying in the unit from urosepsis with no imaging. Step 1 is source control, and without imaging you're just assuming you have that with antibiotics alone.