Depends on the cat. I did a comparison between PY (simple DPN number), Texel and SCHRS based on the following assumption: A race where an F18 sails 45 minutes real time. What real time needs to sail a H16, an A cat (Flyer in SCHRS) and a Tornado to get the SAME corrected time in the different rating systems?

Diff_F18 PY2006 Texel SCHRS
Hobie 16 588.5 427.7 378.8
Tornado -147.1 -187.1 -188.1
A class 95.2 0.0 -69.9

(difference in real time with respect to F18, negative means he has to finish earlier, positive later, units is seconds)

I.e. in PY a Hobie 16 has to cross the line 588 seconds after the F18, whereas in SCHRS he has to finish only 379 seconds after the F18. The difference is 210 seconds or 8% which is the biggest deviation in this table. Between Texel and SCHRS the deviations are smaller, a H16 has 50 seconds more with Texel which is 2%.

Of course one should better apply the BF numbers for PY, but this calculation gives an idea about the ballpark. At say 10 knots boat speed which is 5 m/s 200 seconds are 1000 meters!