For a given cylinder head design, plug heat range is primarily a function of how much heat is released in each cylinder. The 2.3 has a larger cylinder than the inline sixes, and on average each 2.3 cylinder has to work at a higher level of power (which means more heat), so that's probably why the OEM plug for the 2.3 is one heat range colder than the sixes - 8 versus 9.
Unless you drive the car real hard, I don't think a "9" is going to be a problem, and a slightly hotter plug will usually reduce oil ash buildup on the insulator that is typical of older engines with higher oil consumption.
In the world of vintage 327 CID Corvette engines, GM always recommended an AC heat range "4" (AC44), but I learned long ago that 44s usually foul rapidly and heat range "5" (AC 45) works best for normal town and highway driving, and this applies to both the medium performance engines (250 & 300 HP) and Special High Performance/Fuel Injection engines (340 to 375 HP). Though the SHP/FI engines have significantly greater output, in normal street driving you can only use that output for a few seconds at a time. One average, in normal driving, all engines were running about the same power/heat per cylinder.
For hot lapping sessions I ran AC 43s with good results and when I ran my 2.6 in the '89 Silver State Classic Challenge, I ran a Champion BN60 racing plug that was probably a bit colder than Bosch heat range "6". For sustained high output you need to run a cold plug. Otherwise, excessive tip temperature can get the engine into preignition, which leads to detonation, and major damage in a hurry at WOT and high revs.
It's always best to err on the cold side because it's much better to foul a set of plugs than burn a piston or valve, but in the case of running a 9 in a street driven 2.3, I don't think it will be a problem.