I bought a nearly new Odyssey battery from this very site for a very reasonable price, but to be honest the bike bike starts just as it ever did. Its a lead acid battery.
If I was buying brand new I would just get an ordinary lead acid for £40 or the opposite extreme for a lightweight LiFePO4 lithium iron phosphate. Does a GS need to save a kg or two? Probably, but not at £50+ a kg. At least Ballistic batts have a long service life, no acid to spill and there's the bling farkle factor.