It would still be wise to check the tuning of the motors when you have installed the new spindle, since although the increase in mass of the gantry is not that great, if the machine is already set close to the limit you could have problems (or vice versa).

Annoyingly the price of these spindles doesn't drop much for the lower power (0.8kW, 1.5kW) spindles, so it's far better value to get a 2.2kW spindle however that might be a bit too heavy for the gantry? You would never actually draw 2.2kW from the spindle, but may still approach it's limit since the power rating is significantly reduced at low speeds. This makes drilling aluminium more difficult for instance if you get a lower power spindle.