Globally KM, currently you can't go past a JPK1080 or the earlier JPK 1010. The custom IRC designs by Jason Ker or Mark Mills are also pretty good and the Cookson 50s are something of a legend for IRC. Most of the European production cruiser / racer designs aren't bad either.
Locally a 1020 or most of the farr designs rate at least okay, always liked the 88 rating though only Flying Machine in Christchurch has taken IRC seriously, Playbuoy did okay under IRC as well.
Guessing though that's not really what you're asking and whether you're wasting your time will depend on the race and who's in it.
In general for a 930, don't bother, you haven't a hope in hell, if you want to win buy a Farr, i'tll be cheaper than the mods to make a 930 better but better is better than a 930 you don't change, not better I'll win.
Comparison, an 88 should rate somewhere between 0.935 and 0.948, a 1020 0.945 to 0.960 while the class standard 930 comes in a little over 0.980. The tweaked ones go as high as 1.05 but no ones got a cert for the most heavily tweaked 930s, The Entertainer, No Worries et al will be higher again. In very specific conditions you may sail to the rating but basically you're praying for conditions where a 930 is planing all day and no one else can, possible but probably happens only one time in 25 depending on the race so not real good odds.
If it's the Solo Tasman, traditionally hasn't had much IRC but looking at the entry list, don't like your chances. Katana (assuming that's ex SF3600) will be very competitive, as will the Sigma 36 and the S&S 34. Zenith should be okay if they get a cert and don't race with their code zero. Most of that fleet shouldn't rate too badly, like the 930s, the Leisure 42 is punnished somewhat by the rule but I suspect she has a lot more scope to successfully optimise if they want to.
Sorry, suspect my comments basically suck from your perspective.