Unfortunately it doesn't work like that
It's just the removal of a speed cap. A bit like some ISPs treat(ed) ADSL2+, where you could be on the ADSL2+ service but have a maximum speed of 8Mbps on their basic package, but up to 24Mbps on their higher priced ones. O2 and Be used to do that on their LLU.
If you were getting the full speed on your 40Mbps one then yes, Infinity 2 would give you whatever your line could do. But as you are only getting 20Mbps, no better than now.
There's a slight difference between the connection speed and the throughput, just like there is on ADSLx, but that isn't enough to matter in your case. You would need to be getting 37Mbps or so on speed tests now.
My broadband basic info/help site - www.robertos.me.uk
Domains,website and mail hosting - Tsohost
. Connection - Plusnet
Extra Fibre (FTTC). Sync ~ 56.0/13.9Mbps @ 600m.
"Where talent is a dwarf, self-esteem is a giant."
- Jean-Antoine Petit-Senn.
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