take the plugs out and also the fuse which powers the coil packs. There's a post somewhere here re. compression numbers, remembering that the low compression engines will produce lower numbers than the HC engines delivered to the USA and reported on in rr.net.
Go faster cams generally have slightly higher lift.. do a search on google for Des Hammill's book re tuning rover V8 engines for track and road etc. There are some sites with extensive extracts which deal with cams/lifters etc. Look for the extract which covers
Chapter 6 on camshafts and valve springs. Once you start playing with high lift cams (e.g. Piper etc) which have 10.9mm (0.43") lift you may incur valve spring binding ... you may be better off looking for a cam which brings the torque on song lower in the rev range... If you need to have the heads planed, then you would be well advised to buy some shims from Turner engineering in the UK to put under the rocker posts to restore the correct preload on the hydraulic lifters. There's plenty of references on how to do this... send me a pm if you get stuck.
Good luck with it.
With some fiddling you can remove the cam with out removing the engine... I've put some photos in a couple of months back when I completed the head gasket overhaul...
PaulP38a 's personal web pages on refreshing one's head 
are immensely helpful
cheers
MY99 RR P38 HSE 4.6 (Thor) gone (to Tasmania)
2020 Subaru Impreza S ('SWMBO's Express' )
2023 Ineos Grenadier Trialmaster (diesel)
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