Is an 8 litre boil too small for a 23 litre extract kit?

I would recommend NOT boiling the malt extract. There is absolutely no point in doing so. The extract is already sterile and at best only needs pasteurisation which can be done by dissolving the extract in several litres of near boiling water in the fermentation vessel when making up the wort. The flavour comes in the main from the steeped grain and hops and the un-fermented malto-dextrins of the malt extract.There is not much point in boiling roast grains either. Just put the grains in a suitable sized pan (or pans) and add boiling water. Allow to stand as preferred but 30 minutes is usually enough. Then strain into the fermentation vessel/bucket. Rinse grains with more hot water.

Many kits come with pre-hopped extract but some come with leaf/pellet hops and the hops usually need boiling. This is simple enough and can be done in a suitable sized pan (8 litres is good) by boiling with water for the required amount of time (eg 1 hour). Any timed hop additions are done in the usual way - just add hops to the boiling water at the required time. At the end of the boil time, strain the "hop tea" into the fermentation vessel and perhaps wash the hops with more boiling water. This is a good time to add the malt extract. When all (inc any sugars/syrops) is stirred in and dissolved, add cold water (and maybe some more boiling water) to reach the required volume and temperature in the fermentation vessel. When all ready and at correct temp - pitch the yeast. I recommend making an active yeast starter solution up with sugar in a suitable bottle some hours before brewing. If the yeast is bubbling then it can be added. If not get some more yeast!

I and others now use this extract method all the time and it produces great beer with less effort, less energy, and more safety. The cooling is done efficiently and rapidly using water (eg from a tap or in a bulk mineral water bottle) Boiling malt can be a dangerous thing, is at best difficult to clean up, is at worse a real scalding danger and for all of that does nothing for the final taste of the beer. Have a go - you won't go back to boiling it.

Most of the extract recipes I've brewed have been 'half-boil' recipes where approximately a third of the LME is added to the steeping liquid plus a few (4-5) extra litres of water for the boil. Boil then proceeds according to recipe, but less volume. The rest of the extract is then added when increasing volume and dropping temp ready for pitching.

Overall, have never had a problem with lack of hops flavour doing it this way.

I'm not sure about splitting a full boil into 2 parts, never tried it!