I work in insurance and deal with a lot of modified land rovers. The type of mods you mention would not usually incur any terms. Anything performance enhancing (ie chips) or anything that will have a significant impact on handling (ie non standard wheels or raised suspension) would incur a premium load.
Modifications will often not increase the market value (and therefore the amount that would be paid out in the event of a total loss).
With regards to your final query – anyone who made a claim and was then found to have modified the vehicle and not informed the insurer would probably find their claim rejected.
Saying all this though, the fact is all insurance companies will likely have different approaches, terms and penalties, so the only way to be sure is to question your insurer.
Hope this helps.