Are there any downsides to a lightened flywheel? I.e less inertia up hills etc?
Only if not on throttle.
I have mine done in my MX-5 as 1.8 one weighed a tonne compared to the 1.6 one i was changing from.
One thing at a time,he is only changing the box,so worthwhile doing.If he was rebuilding the engine then yes as you suggest.
Be careful to tell whoever does this work what you are revving to as some company's will take too much off and then Bang.
well i was looking into the ktec fly wheel they sell as its a straight swap and its already light
Getting you own done will be £40-100
Buying one will be £200+