ASIC Hardware Development of the Future

[maxx2097]

We all know that ASICs changed the whole BTC business. Hashrate and difficulty are on an exponential growth.
The current hardware can do about 1GH/Watt (KNC... and nearly 2GH/Watt anounced by BFLs 28nm miner). Is this already the reachable maximum, or can future hardware improve the GH/Watt ratio even more?
My opinion is that ASIC hardware development will stall leading to a more linear hasrate/diffculty increase.