Paradox: Higher budget is bad for your Adwords campaign

I have been running some AdWrods campaigns for more than 2 years and I have a lot of historical data, through out this period I reached what you can say the optimal CPC for each Ad Group that gives me highest conversions within my budget.

My budget was small so my CPC values were low accordingly, now I have much higher budget (10x) available to spend, so I started testing to see what I can achieve with this. So basically I expect to get 10x more conversions when I spend 10x more, right?

wrong!

Higher Bidding led to lower conv rates

While I was testing with increasing my CPC bidding, I noticed that the higher the CPC the lower conversion rates become, you get more impressions and higer CTR but you get lower conversions,I am not sure if that is because Pos#1 is not ideal for conversions (if that is true then it will be problematic because it means you should never increase your bids to avoid being in #1).

The issue is not related to budget setting, because I increases the daily budget for each campaign when I increased my bids , also when I look at Lost Search Imp (Budget) it is 0.00% all the time.

Also my quality scores are very good, I have plenty of 10/10 and 9/10 keywords, and rarely below 7/10 keywords.

Higher bidding led to lower share impressions!

The second strange thing now is that I tried to increase my impressions shares in 2 campaigns,

So 3 days ago I set the bids 3x times higher and enabled Enhanced CPC, guess what happened? The next day my impression shares dropped from 50% to 35% (I will give it more time maybe 3 days not enough to tell)

I am completely baffled!

Where, how to spend my budget?

This is very depressing the high budget now I have is useless, although I can spend now 10x times, but I will not get more conversions, and even If I manage to get more conversions, my cost / conv will be insanely higher than my target CPA.

Edit: I have new theory, which is that Google uses your average monthly spending as a hidden cap for your campaigns budget, thats the only explanation I could think of to explain why impressions share decreases when I increase CPC regardless of any increase in the budget (even up to 10x)

Any advice?
thanks