Would it not matter because it’s close enough?
Would you not make gains because it’s not truly in the right zone?
Would you complete more workouts because they would be more doable and therefore make even more gains?
What if it were 10% lower? All hypotheticals!
You’d be doing 5% less work each week, so you’d lose fitness or at least gain fitness more slowly than you otherwise would have.
That’s assuming that your FTP and workload were at the right level to stimulate steady and sustainable fitness improvements. If lowering your FTP 5% enables you to complete significantly more workouts, then that means you were previously failing a significant number of workouts, which in turn means your FTP, volume and/or recovery weren’t right.
I guess this thread might help you. You are looking 5% to even more 10% reduction in your current “set” FTP value… Then it’s probable that you have over estimated ftp.
Can the ramp test overestimate FTP
Has my ramp test over estimated my FTP?
I think you’d hardly notice 5%. Training stimulus is like a dial switch. %5 is not a huge difference. It’s not so much like a light switch where after you get to some magical wattage things happen.
If you went a larger percentage like 10%, the down side might be trying to train above FTP but actually being below it.
Chad always says in the podcast that it’s not problem to dial it back or to back pedal to complete a workout. People seem to have trouble with these instructions!
At my threshold a 5% decrease is just about 18 watts. If I lowered all my intervals by that much I feel like I would probably be not training hard enough for the systems I want to be hitting, unless it was done 2 riding and it was on the high end.
@wildone It’s the opposite. The ramp test underestimates my FTP. I’m just curious what others think if this were the case for them. It’s really difficult to even know truly. The only reason I know it’s at least 5% higher is because I was able to hold that 5% higher amount for an hour a week prior.
@JT_34 yeah, I was thinking for higher thresholds the watt difference would be much greater. For me it’s barely over a 10 watt difference at 5%, so I feel like that seems much less drastic. And as @AJS914 states, it’s not one zone or another that you are training, there is overlap.
Funny that you said that; I just (as in a few hours ago…) listened to the FasCat podcast where Frank Overton discusses sweet spot training with Dr. Andy Coggan. He (Andy) used the term “dimmer” to illustrate how the different energy systems activate and overlap each other.
Just as with the concept of sweet spot a lower intensity enables you to do more volume, which imo basically is a good thing 'cause it is easier to recover from.
ah sorry, I misread the title 
Disclaimer: IANAE (I am not an expert)
The impact will also vary greatly depending on your current fitness/level of training. If you are completely untrained you would make progress training at any % of your FTP (other than 0!). If you are highly trained / near your peak potential fitness, you may not make any improvements or may lose fitness. If you have peaked/tapered you will lose fitness even training at 100% of your FTP.
I’m not sure that training at 100% of your FTP is optimal for every individual (depending how their body responds to training stress) - but I’m completely speculating here.
TLDR: Unless you’re highly trained you will gain fitness at 5% of your FTP, but possibly slower (and maybe even faster).