Although expensive, the Radikal is a great upgrade. If you can only afford one upgrade, Radikal before Keel for sure.
My friend has a full spec's LP12 and tried both the Akiva and Kleos, he liked both cartridges but settled on the Akiva due to the Linn match.
I would give the Kleos a serious listen. I would imagine that the Lyra provides more value than the Linn due to Linn's huge mark ups.